computer Related Articles

What is Hard Drive?Hard disk, hard disk drive (HDD), hard disk, or fixed disk is the most important storage device of computer. It is composed of one or more aluminum or glass discs. These discs are covered with ferromagnetic material. Your documents, pictures, music, videos, programs, application preferences, and operating system represent digital content all can be stored on a hard drive. Most hard drives are permanently sealed and fixed in the hard drive. Early hard disk storage media was replaceable, but today's hard disk is a fixed storage medium, which is sealed in a case. With the development, removable hard disks have also appeared, and they are becoming more and more popular with different types. In addition, most of the hard disks installed on microcomputers are called Winchester hard drive.Hard DriveTopics Covered in this GuideWhat is Hard Drive?Hard Drive Tech ParametersHard Drive ClassificationsHard Disk StructureHow Does Hard Disk Work?Hard Drive MaintenanceUsing TipsHard Drive FaultsTypes of Hard Disk InterfacesCan a Computer Run without a Hard Drive?Should I Buy SSD or HDD?What the Defferences between HDD and SSD?What is the Lifespan of a Hard Drive?How Much Do Hard Drives Cost?What are the Best Hard Drives?Hard Drive Tech Parameters1) HD CapacityCapacity is the most important parameter of the hard disk. The capacity of the hard disk is measured in megabytes (MB) or gigabytes (GB). However, the hard disk manufacturer usually takes 1G=1000MB in the nominal hard disk capacity, so the capacity we see in the BIOS or when formatting the hard disk will be smaller than the manufacturer’s nominal value.The hard disk capacity index also includes the single-disk capacity. It refers to the capacity of a single disk of a hard disk. The larger the single disk capacity, the lower the unit cost and the shorter the average access time. For users, the capacity of the hard disk is like the computer memory, and it will never be too much.2) Rotational SpeedRotational speed, or spindle speed is the rotation speed of the motor spindle in the hard disk, which is the maximum number of revolutions that the hard disk platter can complete in one minute. It is one of the important parameters indicating the grade of the hard disk. It is one of the key factors that determine the internal transmission rate of the hard disk, and directly affects the speed of the hard disk to a large extent. The faster the rotation speed of the hard disk, the faster the hard disk can find files. The hard disk speed is expressed in revolutions per minute, and the unit is expressed as RPM(revolutions per minute). The larger the RPM value, the faster the internal transfer rate, the shorter the access time, and the better the overall performance of the hard drive.The spindle motor of the HDD makes the platters to rotate at a high speed, generating buoyancy to make the head float above the platters. Bring the sector of the data to be accessed below the head, the faster the speed, the shorter the waiting time. Therefore, the rotational speed largely determines the read speed of the hard disk.The rotation speed of ordinary hard disks for household use is generally 5400rpm and 7200rpm. High-speed hard disks are the first choice for desktop users. For notebook users, it is mainly 4200rpm and 5400rpm. Although some companies have released 7200rpm notebook hard disks, they are still rare in the market. Users have the highest requirements on server for hard disk performance. The speed of SCSI hard disks used in servers is basically 10000rpm, and even 15000rpm. The performance is much higher than that of household products.A higher speed can shorten the average seek time and actual read and write time of the hard disk. However, as the speed of the hard disk continues to increase, it also brings negative effects such as temperature rise, heavy motor spindle wear, and great operating noise. The speed of notebook hard disks is lower than that of desktop hard disks, which is affected to a certain extent by this factor. The internal space of the notebook is small, and the size of the notebook hard disk (2.5 inches) is also designed to be smaller than that of the desktop hard disk (3.5 inches). The temperature rise caused by the increase in speed puts higher requirements on the heat dissipation performance of the notebook itself. In addition, the noise becomes larger, it is necessary to take noise reduction measures, so more technique requirements on the notebook hard disk productions. At the same time, the increase in speed, while the others remain unchanged, means that the power consumption of the motor will increase, the more electricity is consumed per unit time, and the working time of the battery is shortened, so that the portability of the notebook will be affected. Therefore, notebook hard drives generally use a relatively low-speed 4200rpm hard drive.3) Access TimeThe average access time refers to the time required for the head to reach the target track position from the starting position and find the data sector to be read and written on the target track.The average access time reflects the read and write speed of the hard disk, which includes the seek time and waiting time of the hard disk, that is, average access time = average seek time + average waiting time.The average seek time of the hard disk refers to the time required for the head of the hard disk to move to the specified track on the disk surface. This time is of course as small as possible. The average seek time is usually between 8ms and 12ms, while a SCSI HDD should be less than or equal to 8ms.The waiting time of the hard disk, also known as the Latency, refers to the time that the magnetic head is already in the track to be accessed and waiting for the sector to be accessed to rotate below the head. The average waiting time is half of the time required for the disc to rotate one round, and should generally be less than 4ms.4) Data Transfer RateData transfer rate refers to the speed at which the hard disk reads and writes data, in megabytes per second (MB/s). It includes internal data transfer rate and external data transfer rate.The internal transfer rate is also known as the sustained transfer rate, which reflects the performance of the hard disk buffer when it is not in use. The internal transfer rate mainly depends on the rotation speed of the hard disk.The external transfer rate is also called the burst data transfer rate or the interface transfer rate. It is nominally the data transfer rate between the system bus and the hard disk buffer. The type of hard disk interface is related to the size of the hard disk cache.The maximum external transfer rate of the Fast ATA interface HD is 16.6MB/s, while the Ultra ATA interface hard disk reaches 33.3MB/s.The hard disk using SATA (Serial ATA) port is also called serial hard disk. Serial ATA adopts a serial connection method. The serial ATA bus uses an embedded clock signal and has a stronger error correction capability. Compared with the past, its biggest difference is that it can check the transmission instructions (not just data). Errors are found to be automatically corrected, which greatly improves the reliability of data transmission. The serial interface also has the advantages of simple structure and support for hot swapping.5) Cache MemoryCache memory is a memory chip on the hard disk controller with extremely fast access speed. It is a buffer between the internal storage of the hard disk and the external interface. Since the internal data transfer speed of the hard disk is different from the transfer speed of the external interface, the cache plays a role as a buffer among them. The size and speed of the cache is an important factor directly related to the transmission speed of the hard disk, which can greatly improve the overall performance of the hard disk. When the hard disk accesses fragmented data, data needs to be continuously exchanged between the hard disk and the memory. With a large cache, the fragmented data can be temporarily stored in the cache, reducing the load on the external system and improving the data transmission speed. Hard Drive Classifications1) Mechanical Hard Disk (HDD)HDDMechanical hard disk (HDD) is a traditional hard disk, one of the main storage media for computers. It is composed of one or more magnetic discs made of aluminum or glass, magnetic heads, rotating shafts, control motors, head controllers, data converters, interfaces and caches. When working, the head is suspended on a high-speed rotating disc to read and write data. Mechanical hard disk is a computer storage device that integrates precision machinery, microelectronic circuits, and electromagnetic conversion.2) Solid State Drive (SSD)SSDA solid state drive (SSD) is an array storage composed of multiple flash memory chips plus a main control and cache, and belongs to a hard drive made of an array of solid electronic storage chips. Compared with a mechanical hard disk, the read speed is faster and the seek time is shorter, which can speed up the operating system startup speed and the software startup speed.3) Solid State Hybrid Drive (SSHD)SSHDSolid state hybrid drive is a combination of mechanical hard disk and solid-state hard disk. It uses small-capacity flash memory particles to store commonly used files. Disk is the most important storage medium. Flash memory only serves as a buffer to reduce seek time and improve efficiency. Hard Disk StructureThe hard disk is one of the most important storage for computers. Most of the software needed for the computer to function properly is stored on the hard drive. Because the storage capacity of hard disk is large, it is different from computer memory and optical disk. Hard disks are storage devices based on hard rotating disks used on computers. It stores and retrieves digital data on a flat magnetic surface.Take a Look Inside a Hard Drive1) Magnetoresistive Heads (MR heads)The magnetoresistive head is the most expensive part of the hard disk, and it is also the most important and critical part of the HD technology. The traditional magnetic head is an electromagnetic induction magnetic head that combines reading and writing. However, the reading and writing of hard disks are two completely different operations to limit the hard disk design. The MR head uses a separate head structure: the write head still uses the traditional magnetic induction head (MR head cannot write), and the read head uses a new type of MR head. In this way, during the design, the different characteristics of the two can be optimized separately to obtain the best read/write performance. In addition, the MR head gets the signal amplitude through changes in resistance rather than changes in current, so it is very sensitive to signal changes, and the accuracy of reading data is correspondingly improved. Further more, because the read signal amplitude has nothing to do with the track width, the HD track can be made very narrow, thereby increasing the density of the disc. In addition, GMR heads (Giant Magnetoresistive heads) made of materials with a multi-layer structure and better magnetoresistive effect have gradually become popular.2) Magnetic TrackWhen the disk rotates, if the head is held in one position, each head will draw a circular track on the surface of the disk, called tracks. They are invisible to the naked eye at all, because they are only some magnetized areas on the disk surface that are magnetized in a special way, and the information on the disk is stored along such tracks. Adjacent tracks are not close to each other. This is because when the magnetization units are too close, the magnetism will affect each other, and at the same time it will also cause difficulties for the magnetic head to read and write. For example, a 1.44MB 3.5-inch floppy disk has 80 tracks on one side, and the track density on the hard disk is much greater than this value, usually there are thousands of tracks on one side.The surface of the disk is coated with a magnetic medium used for recording, which are magnetic particles under the microscope. The polarity of tiny magnetic particles can be quickly changed by the magnetic head, and can be maintained stably after the change. The system distinguishes 0 or 1 in the binary system through changes in magnetic flux and magnetoresistance. It is precisely because all operations are performed under microscopic conditions, so if the hard disk is operated at high speed while being shocked by external forces, it may cause irreversible data loss due to the head slaps on the surface of the disk. In addition, the uniaxial anisotropy and volume of the magnetic particles will obviously affect the thermal stability of the magnetic particles, and the thermal stability determines the stability of the magnetic particle, that is, the correctness and stability of the stored data. However, it cannot be increased blindly, because it is limited by the write field that the magnetic head can provide and the signal-to-noise ratio of the medium.3) FanEach track on the disk is equally divided into several arc segments, which are the sectors of the disk. Each sector can store 512 bytes of info. The hard disk drive reads and writes data to the disk  from the sectors. 4) CylinderA hard disk is usually composed of a set of overlapping disks. Each disk surface is divided into an equal number of tracks, and numbered from the "0" on the outer edge. The tracks with the same number form a cylinder. The number of cylinders on a disk is equal to the number of tracks on a disk. Since each disk surface has its own head, the number of disk surfaces is equal to the total number of heads. The so-called CHS of the hard disk, namely Cylinder, Head, Sector. So when the number of CHS of the hard disk is known, the capacity of the hard disk can be determined. The capacity of the hard disk is the number of cylinders and the number of sectors. How Does Hard Disk Work?How Do Hard Drives Work?When the hard disk is working, never turn off the power forcibly, which will cause physical damage to the hard disk and data loss. In addition, with high-speed components in the hard disk, if the high-speed disk is shut down forcibly and suddenly, which is more likely to cause damage to the hard disk. So don't turn on the computer immediately after shutting down. This requires time buffering.When the hard disk is working, try to avoid its vibration, because the distance between the magnetic head and the magnetic disk is very close. If it is subjected to severe vibration, the magnetic head will hit the magnetic disk to damage it, which will make the entire hard drive unusable.In the process of using the hard disk, many users compress files to reduce the use of disk space. This will cause the compressed volume file to continue to grow. The data access speed also slowed down, and the number of reads and writes increased, which would affect the heat generation and stability of the hard disk, even reduce service life.  Hard Drive MaintenanceFirst of all, the impact of dust on the hard disk is not small. If dust is attracted to the circuit board, it will cause unstable operation of the hard disk or damage to internal parts. The functional working status of the hard disk has a great relationship with the temperature. Too high or too low temperature will cause the clock frequency of the crystal oscillator to change, which will cause the circuit components to malfunction. In addition, if the temperature is too low, it will cause the air moisture condenses on the component, causing a short circuit.Second, we need to clear your hard drive regularly. This will increase the speed of your hard drive. If there are too many junk files on the hard disk, the speed will slow down and the tracks may be damaged. However, clean up frequently will also reduce the service life of the hard drive.Finally, it is anti-virus. Viruses are the biggest threat to the files stored on the hard drive. Therefore, once we found that the virus should be cleared up in time and try not to format the hard disk. Using Tips1) Don't shut down suddenly while working.When the hard disk starts to work, it is generally in high-speed rotation, if we suddenly turn off the power in the middle, it may cause violent friction between the head and the platter to damage the hard disk. Therefore, it is necessary to avoid this. When shutting down, you must pay attention to whether the hard disk indicator on the panel is still flashing, only the indicator stops flashing and then hard disk read and write ends, you can turn off the computer.2) Prevent dust from entering.Dust can cause great damage to the hard disk. This is because in a severely dusty environment, the hard disk can easily attract dust particles in the air, causing them to accumulate on the internal circuit components of the hard disk for a long time, which will affect the heat of the electronic components, causing the temperature of the circuit components to rise, and resulting in leakage or burnout of the components.In addition, dust may also absorb moisture, corrode the electronic circuits inside the hard disk, and cause some invisible problems. Therefore, although the volume of dust is small, the harm to the hard disk cannot be underestimated. Therefore, it is necessary to maintain environmental sanitation and reduce the humidity and dust content in the air. In addition, users cannot remove the hard disk cover by themselves, otherwise the dust in the air will enter the hard disk and scratch the platters or heads during read and write operations.3) Temperature ControlAs we all known, temperature affects the service life of the hard disk. A certain amount of heat is generated when the hard disk is working, so there is a heat dissipation problem during use. 20～25℃ is better. Temperature can also cause failure of hard disk circuit components, and magnetic media can also cause recording errors due to thermal expansion.On one hand, when the humidity is too high, the surface of the electronic components may absorb a layer of water, oxidizing and corroding the electronic circuits, resulting in poor contact or even short circuits, and it will also cause the magnetic force of the magnetic medium to change, causing data reading and writing errors. On the other hand, it is easy to accumulate a large amount of static charge generated by the rotation of the machine in low temperature, which will burn out the CMOS circuit, attract dust and damage the head and scratch the disk. In addition, try not to make the hard disk close to strong magnetic fields, such as loud speakers, motors, radios, mobile phones, etc., so as to prevent the data recorded on the hard disk from being damaged due to magnetization.Hard Disk Drive StructureHard Drive Faults1) HD Cooling FanConsidering the heat dissipation effect, many people install hard drive cooling fans for their computer hard drives. However, some low-end fans have obvious vibrations and can transmit vibration to the hard drive. In the long term, it will definitely affect the life of the hard drive.2) Optical DriveThe reading speed of mainstream optical drives has reached more than 50 times speed. When the optical disc rotates at a high speed, the vibration of the optical drive itself will drive the resonance of the chassis, which affects the work of the hard disk. And this kind of high-speed rotation generates a lot of heat, because the optical drive is so close to the hard drive, the heat released from the optical drive will surely increase the temperature of the hard drive.3) Static ElectricityIn the process of repairing the computer, many people hold the hard disk with their hands, but in dry weather, tens of thousands of volts of static electricity may accumulate on the hands of people, which may break down the chips on the circuit board, causing the hard disk to malfunction.4) FormattingIf the computer hard disk has bad sectors, many users will take formatting measures. In fact, low format damages the hard disk greatly. It may cause the proliferation of bad sectors on the disk, and even cause the loss of hard disk parameters, making the hard disk unable to use.5) Power SupplyA low-quality computer will cause the hard drive to be disturbed by voltage fluctuations, especially when the hard drive is reading and writing. If there is a problem with the power supply, a hard drive can be scrapped in an instant.6) Magnetic FieldBecause the hard disk is a device that relies on magnetic media to record data, if it is interfered by the magnetic field of the external environment, it is likely to cause the loss of disk data, so you should try to stay away from the magnetic field environment. Types of Hard Disk InterfacesThere are five categories of hard disk interfaces: IDE, SATA, SCSI, SAS, FC1) IDE (Integrated Drive Electronics)IDE refers to the hard disk drive that integrates the controller and the disk body, and is a hard disk transmission interface. There is another name called ATA (advanced technology attachment).2) SATA (Serial ATA)SATA hard disk is called serial hard disk based on its serial data transmission method. In the process of data transmission, the data line and the signal line are used independently, and the transmission clock frequency remains independent. Therefore, compared with the previous PATA, the transmission rate of SATA can reach 30 times that of parallel. It can be said that SATA technology is not an improvement of PATA technology in a simple sense, but a new bus architecture.3) SCSI (Small Computer System Interface)SCSI invention is mainly because the hard disk speed of the original IDE interface is too slow. In fact, SCSI is not designed specifically for hard drives, in fact it is a bus-type interface, working independently of the system bus.4) SAS (Serial Attached SCSI)SAS is serial attached SCSI, which is a new generation of SCSI technology. Like the popular SATA hard disks, it uses serial technology to achieve higher transmission speeds, and improves internal space by shortening the cable. It is a brand new interface developed after the parallel SCSI interface, which is designed to improve the performance, availability, and expandability of the storage system, and provide compatibility with SATA hard drives.SAS interface tech can be backward compatible with SATA. Specifically, the compatibility of the two is mainly reflected in the compatibility of the physical layer and the protocol layer.5) FC (Fibre Channel)Just like the SCIS interface, FC is not an interface technology designed and developed for hard disks at first. It is specially designed for network systems. However, as storage systems require high speed, they are gradually applied to hard disk systems. FC hard disk was developed to improve the speed and flexibility of multi-disk storage system. Its appearance greatly improves the communication speed of multi-disk system, and it uses optical cable connections between systems in a point-to-point (or switching) configuration. There are something to note: the hard disk itself does not have an FC interface, where the cabinet has, which is interconnected with an optical fiber switch. Note: Get more info from Types of Hard Disk Drive Interface.Can a Computer Run without a Hard Drive?A computer can still function without a hard drive. This can be done through a network, USB, CD, or DVD. Computers can be booted over a network, through a USB drive, or even off of a CD or DVD. When you attempt to run a computer without a hard drive, you will often be asked for a boot device. Should I Buy SSD or HDD?1) According to data read and write speedA computer with the same configuration can reach a read and write speed of about 500M/S with a solid state drive, but about 150MB/S with a mechanical hard drive. The difference is nearly three times the speed, which makes the difference in computer response speed even greater.2) According to data security and shock resistanceSince the mechanical hard disk reads and writes data through the magnetic head to read the disk, it is easy to cause data damage due to the collision of the disk and the magnetic head during high-speed rotation, especially it is in the handling process that the disc may be damaged, so everyone needs to be extremely careful when touching it.3) According to weight and volumeCompared with mechanical hard disks, solid state drives are smaller and lighter in appearance, and has stronger performance and faster transfer speed than mechanical hard drives.4) According to noise and heat dissipationSince the solid state drive is made of flash memory particles, it is not equipped with mechanical parts and flash memory chips, and there is no disk and head mechanical motors, fans, etc., so that it can ensure absolute silence. The heat is also very small, and the heat dissipation is also very fast.5) According to power consumptionSSDs commonly use less power and result in longer battery life because data access is much faster and the device is idle more often. With their spinning disks, HDDs require more power when they start up than SSDs. For example, the general full-speed power consumption of a 3.5-inch mechanical hard disk is about 12W, and a 2.5-inch hard disk is only about 5W. The full-speed power consumption of the solid-state drive is about 10W, its working power is generally 2-3W, less than 1W in standby mode.Although solid state drives are definitely faster than mechanical hard drives from above mentioned, but it doesn’t mean that solid state drives are necessarily better than mechanical hard drives, because in terms of price and capacity, mechanical hard drives are "T" is the unit, and most of the solid-state drives are still in G. Although there are also "T", the price is beyond everyone's expectations. One more thing to note is that it is more difficult to restore data if the solid state drive is damaged, while the mechanical hard drive can restore data through repair. Therefore, in terms of data security, mechanical hard drives have more advantages in storing important data. In short, consider comprehensively according to the actual situations. What the Defferences between HDD and SSD?A hard disk drive (HDD) is a traditional storage device that uses mechanical platters and a moving read/write head to access data. A solid state drive (SSD) is a newer, faster type of device that stores data on instantly-accessible memory chips.Generally, SSDs are more durable than HDDs in extreme and harsh environments because they don't have moving parts such as actuator arms. SSDs can withstand accidental drops and other shocks, vibration, extreme temperatures, and magnetic fields better than HDDs.SSDs commonly use less power and result in longer battery life because data access is much faster and the device is idle more often. With their spinning disks, HDDs require more power when they start up than SSDs. However, when not in use, magnetic drives are more reliable for long-term storage than flash memory ones. Thus, HDDs are more capable of long time storage than SSDs when powered off.While normal HDDs can last about 10 years max in reality, and an SSD lifespan has a built-in time of death. To keep it simple: an electric effect results in the fact that data can only be written on a storage cell inside the chips between approximately 3,000 and 100,000 times during its lifetime.As for price, SSDs are more expensive than hard drives in terms of dollar per gigabyte. A 1TB internal 2.5-inch hard drive costs between $40 and $60, but as of this writing, the very cheapest SSDs of the same capacity and form factor start at around $100.With their ruggedness and low energy consumption, SSDs are becoming more popular with portable PCs. With all the advantages that SSD has over HDD, price, availability and capacity are probably the primary factors constraining the acceptance of this new technology. Note: Get more info from SSD vs HDD: Is an SSD Really Necessary?What is the Lifespan of a Hard Drive?Though the average might be three to five years, hard drives can theoretically last much longer (or shorter, for that matter). If a hard drive works 24 hours continuously, it will be damaged in less than 3 years. After normal use, there should be no problem for 5 or 10 years. During use, the garbage must be cleaned regularly and kept HD cool, so as not to get stuck.As with most things, if you take care of your hard drive, it will better last to its potential. How Much Do Hard Drives Cost?A 1TB internal 2.5-inch hard drive costs between $40 and $60, but as of this writing, the very cheapest SSDs of the same capacity and form factor start at around $100. That translates into 4 to 6 cents per gigabyte for the hard drive versus 10 cents per gigabyte for the SSD.According to market trend, the number of hard drives sold each year has declined recently due to the migration of consumer PCs to SSDs, and also demand for higher-capacity HDDs by exascale datacenters. When demand for HDDs spikes, retailers sell out quickly, and prices increase as dealers come into play. What are the Best Hard Drives?Best Hard Drives at a Glance1️⃣Seagate BarraCuda2️⃣Toshiba X3003️⃣WD VelociRaptor4️⃣WD Blue Desktop5️⃣Seagate Firecuda Desktop6️⃣Seagate IronWolf NAS7️⃣Seagate FireCuda Mobile8️⃣WD My Book9️⃣G-Technology G-Drive

IntroductionAs everyone knows, in order to create a passive low pass filter, combing resistive elements with reactive elements happens often. Put simply, a typical circuit composed of resistors and capacitors or inductors. According to theories, the resistor–inductor (RL) low-pass topology is equivalent to the resistor-capacitor (RC) low-pass topology in terms of filtering capability. However. in fact, RC low pass filters are more common, so this article will focus on first-order RC low pass filters.In this video, Passive RC Low Pass Filter has been discussed. CatalogIntroductionⅠ Typical RC Circuit1.1 Time Domain1.2 Frequency DomainⅡ First-order Low Pass Filter on Software2.1 Basic Filtering Algorithm2.2 Basic Algorithm of First-order RC Digital FilteringⅢ Optimization Method- Filtering Coefficients AdjustmentⅠ Typical RC CircuitThe RC circuit has thousands of uses and is a very important circuit to study. Not only can it be used to time circuits, it can also be used to filter out unwanted frequencies in a circuit and used in power supplies, like the one for your computer, to help turn ac voltage to dc voltage.Figure 1. Typical RC Circuit (DC, AC, and Pulse Signals can all use it)1.1 Time DomainCapacitor Current:According to Kirchhoff’s Voltage Law:Where, the unit of Ui is volts, the unit of RC is seconds, and τ=RC, get:Suppose the initial voltage of the capacitor is 0, where:R=1000ΩC=4.7uFUi=1Vt=0.0001~0.1sτ=RCVc(τ)=0.632 Figure 2. Step Response Curve of a First-order RC System1.2 Frequency DomainTaking the capacitor voltage as the output, the network function of the circuit is:Where u1=Ui, u2=UoLet ωc be equal to:, which is the cut-off frequency.Amplitude and phase angle function:Value of variables:R=1000ΩC=4.7uF |A(fc)|=0.707θ(fc)=-45, f=0.001, 1, …….100000.Amplitude and phase frequency characteristics:Figure 3.Figure 4.Logarithmic representation of amplitude-frequency characteristic:Figure 5.Analysis:When ω<ωc, the amplitude is a straight line parallel to the coordinate, and there is no attenuation.　When ω>ωc, it is a straight line whose slope is proportional to -20dB/decade.When ω=ωc, the gain is attenuated to 0.707, which is -3dB, and the phase lags by 45 degrees, corresponding to a low-pass filter. This frequency is usually called the cutoff frequency. Disadvantages:When using this analog filter to suppress low-frequency interference, the filter is required to have a larger time constant and a high-precision RC network. Increasing the time constant requires increasing the value of R, and meanwhile, the leakage current increases accordingly, thereby reducing the filtering effect.Figure 6. RC CircuitⅡ First-order Low Pass Filter on SoftwareAdvantages1) The use of digital filtering algorithms to achieve dynamic RC filtering can well overcome the shortcomings of analog filters.2) This kind of algorithm is more practical when the simulation constant is required.3) It has a good inhibitory effect on periodic interference.4) Save RAM space Disadvantages1) Exit phase lag, resulting in low sensitivity.2) It cannot filter out interference with a frequency higher than half of the sampling frequency (called the Nyquist frequency. For example, if the sampling frequency is 100 Hz, it cannot filter out interference signals above 50Hz). In this case, an analog filter should be used.3) For the single-chip microcomputer without multiplication and division running instructions, the workload of the program operation is relatively large.2.1 Basic Filtering AlgorithmOrigin of the AlgorithmThe transfer function of the first-order RC low-pass filter in the S domain for frequency analysis:Through z-transformation (there are many methods, such as first-order forward difference, bilinear transformation, etc. Here, the first-order backward difference method is used): Into the S-domain Transfer Function After the derivation is transformed into the difference equation, we can get:The transfer function in the S domain can be transformed into a difference equation in the time domain through the Z transformation.2.2 Basic Algorithm of First-order RC Digital FilteringX is the input, Y is the output value after filtering, then: a is a parameter related to the RC value, called the filter coefficient, its value determines the weight of the new sample value in the filtering result of this time, and its value is usually far less than 1, when the sampling interval t is small enough:1) The smaller the filtering coefficient, the smoother the filtering result, but the lower the sensitivity.2) The larger the filtering coefficient, the higher the sensitivity, but the more unstable the filtering result.3) The output value this time mainly depends on the last filtered output value, and the current sampled value has a relatively small effect on this output, which plays a corrective role.4) Cutoff frequencyFor example: t=0.5s (f=2Hz), a=1/32where fl=(1/32)/(2*3.14*0.5)=0.01Hz Basic ProgramWrite the program according to the basic principles and formulas of first-order filter, as follows:/*In the program, integer arithmetic is faster than decimal arithmetic. In order to speed up the processing speed of the program, for calculation convenience, a is an integer (from 0~255), 1-a is replaced by 256-a, which means that the new sample value is being filtered. The weight in the result (you can also change the base of 1-a to 100-a, and the calculation result will be processed accordingly)*/#define a 128 char value; //Last filtering valuechar filter(){    char new_value;    new_value=get_ad();//Sampling value    return(256-a)*value/256+a*new_value/256；}Initial Optimization of the ProgramReduce the number of operations of multiplication and division to increase the speed of operations.Specific optimization methods:First compare the new sampled value with the previous filtering result, and then use different formula calculations based on the comparison, so that the calculation efficiency of the program is doubled.Resolve the basic formula to get: ProcessNotes:S → New Sampling ValueR → Previous Filtering ResultC→ Filter CoefficientN→ New Filtering Result Program/*Int: NEW_DATA     New sampling values       OLD_DATA       Last filtering result       k        Filter coefficient (0~255)  Out:         The filtering results */ char filter_1(char NEW_DATA,char OLD_DATA,char k){    int result;    if(NEW_DATA<OLD_DATA)    {        result=OLD_DATA-NEW_DATA;        result=result*k;        result=result+128;//+128 Round Up        result=result/256;        result=OLD_DATA-result;    }    else if(NEW_DATA>OLD_DATA)    {        result=NEW_DATA-OLD_DATA;        result=result*k;        result=result+128;//+128 Round Up        result=result/256;        result=OLD_DATA-result;    }    else result=OLD_DATA;    return((char)result);} Filtering AnalysisWhen the filtering coefficient is 30:Figure 7.When the filtering coefficient is 128:Figure 8.When the filtering coefficient is 200:Figure 9.It can be seen that the smaller the filtering coefficient, the smoother the filtering result, but the lower the sensitivity. On the contrary, the larger the filtering coefficient, the higher the sensitivity, but the more unstable the filtering result.Insufficient1) The contradiction between sensitivity and smoothness2) Errors caused by discarding decimals.For example: the current sampling value=25, the last filtering result=24, and the filtering coefficient=10;According to the algorithm, the filtering result of this time = 24.0390625In single-chip microcomputers, floating-point numbers are rarely used, and the fractional part is either discarded or needs to round up. In this way, the result is 24. If the sampling value is always 25, the result will always be 24. Because the filtering result and the actual data will always have an error that cannot be eliminated. Sometimes it will cause the filtering result curve to deviate from the actual value when the sampling data is stable at a certain value (that is, there is a large error between the filtering result and the actual result although in a stable case). Be Careful1) Changing the filtering coefficient, increasing it will reduce the smoothness, and if it is too large, the filtering will lose its meaning.2) The use of decimal part in calculations will bring heavy computational pressure to the CPU. Ⅲ Optimization Method- Filtering Coefficients AdjustmentRealize the Function1) When the data changes rapidly, the filtering results can be followed up in time, and the faster the data changes, the higher the sensitivity should be (sensitivity priority principle).2) When the data becomes stable and oscillates within a range, the filtering result can become stable (the principle of stability first).3) When the data is stable, the filtering result can be approximated and finally equal to the sampling data (eliminate the error caused by decimals in the calculation). Judgment before Adjustment1) Whether the data changes consistently. For example, when the two consecutive sampling values are larger than the previous filtering result, it is normal, otherwise it is regarded as inconsistent.2) Whether the data changes quickly, which is to judge the difference between the sampling value and the previous filtering result.Adjustment Principle1) When the two data changes are inconsistent, it means there is jitter. Clear the filtering coefficient to zero, and delete the new sampling value.2) When the data changes consistently, gradually increase the filtering coefficient to provide the weight of this sampling.3) When the data changes quickly (difference value> debounce count acceleration response threshold), the filtering coefficient should be increased quickly. Adjusting Filter Coefficient Process① Calculate the difference (absolute value) between the current sampling value and the last filtering result; Set the data change direction flag.② Two changes in the same direction?③ First order filter coefficient + coefficient increment (the maximum value is taken when the result is greater than the maximum value). Several Constant Parameters and Their Ranges1. Debounce counting acceleration response threshold is determined according to the actual situation.2. The maximum value of debounce count, which is generally 10.3. The increment of filtering coefficient range is 10~30.4. The maximum value of the filtering coefficient is generally 255.Before starting the first-order filtering program, open the adjustment filter coefficient program to adjust the coefficients in real time. Filtering Effect1. When the sampled data is accidentally interfered, the interference in the filtering result is completely filtered out.2. When the data oscillates within a range, the filtering result curve is very smooth, almost a straight line.3. When the sampling data has real changes, the filtering results can be followed up in a relatively timely manner.4. When the sampling data becomes stable, the filtering result gradually approaches and is finally equal to it.Finally, improve the algorithm. Taking into account the requirements of sensitivity and stability; and meanwhile, it does not consume too much RAM space. As long as a few constants are adjusted reasonably, the algorithm is more suitable for practical applications. Frequently Asked Questions about RC Low Pass Filter1. What is RC low pass filter?A low pass filter is a filter which passes low-frequency signals and blocks, or impedes, high-frequency signals. ... Low pass filters can be constructed using resistors with either capacitors or inductors. A low pass filter composed of a resistor and a capacitor is called a low pass RC filter. 2. Why RC circuit is low pass filter?Then by carefully selecting the correct resistor-capacitor combination, we can create a RC circuit that allows a range of frequencies below a certain value to pass through the circuit unaffected while any frequencies applied to the circuit above this cut-off point to be attenuated, creating what is commonly called a rc low pass fiter. 3. What is difference between RC low pass filter and RC high pass filter?Low pass filter is the type of frequency domain filter that is used for smoothing the image. It attenuates the high frequency components and preserves the low frequency components. High pass filter: ... It attenuates the low frequency components and preserves the high frequency components. 4. What is the transfer function of a low pass filter?Low Pass Filters and their Transfer FunctionsAs its name implies, a low pass filter is an electronic device that allows low frequency AC signals to pass a current through the filter circuit. The output from the filter circuit will be attenuated, depending on the frequency of the input signal. 5. How is low pass filter frequency calculated?The cut-off frequency or -3dB point, can be found using the standard formula, ƒc = 1/(2πRC). The phase angle of the output signal at ƒc and is -45o for a Low Pass Filter.

IntroductionIn-memory computing (IMC), a technique of future computing, stores data in RAM to run calculations entirely in computer memory. With the rise of the big data era, faster data processing capabilities are required. Computer memory and storage space are also growing exponentially to adapt to large-capacity data collection and complex data analysis, which promotes the development of AI (artificial intelligence), and then derives emerging stuff, that is, in-memory computing.In-memory Computing (IMC) ExplainedCatalogIntroductionⅠ Memory Wall: Processor /Memory Performance GapⅡ Developing RequirementⅢ What Is In-memory Computing?3.1 In-memory Computing Definition3.2 Four Realization MethodsⅣ Driving Force of In-memory Computing and Market Prospects4.1 In-memory Computing for AI4.2 In-memory Computing Product Outlook4.3 In-memory Computing Market and ProspectⅤ ConclusionⅠ Memory Wall: Processor / Memory Performance GapThe von Neumann architecture has occupied the dominant position in computer system when the computer invented. This kind of calculation method is to store the data in the main memory first, and then fetch the instructions from the main memory to execute them in order when running. We all know that if the connecting speed of the memory cannot keep up with the performance of the CPU, the computing will be limited. This is a memory wall. At the same time, in terms of efficiency, the von Neumann architecture also has obvious shortcomings. It consumes more energy to read and write data than to calculate once time.Figure 1. Von Neumann Architecture DiagramThe performance of computer processors has developed rapidly based on Moore's Law, and has been directly improved with the invention of transistors. The main memory of the computer uses the DRAM. It is a high-density storage solution based on capacitor charging and discharging. Its performance (speed) depends on two aspects, namely the reading/writing speed of the capacitor charging and discharging in the memory and the interface bandwidth between the devices. The read/write speed of capacitor charging and discharging has increased with Moore’s Law, but the speed is not as fast as the processor. In addition, the interface between DRAM and the processor is a mixed-signal circuit, and its bandwidth increasing speed is mainly restricted by the signal integrity of the traces on the PCB. This has also caused the performance improvement of DRAM to be much slower than that of the processor. At present, the performance of DRAM has become an huge bottleneck of overall computer performance, the so-called "memory wall".  It blocks the computing performance improvement.Figure 2. Moore's Law Effect Ⅱ Developing RequirementIn the current AI technology, with the increasing amount of data and calculations, the original von Neumann architecture is facing more and more challenges. Rely on expanding CPU, the hardware architecture can’t have a large amount of calculation. Also the larger storage capacity is heavily rely on the past architecture, it is also very unsuitable for AI. When the memory capacity is large to a certain extent, it can only show that certain technologies need innovation. In order to solve the "memory wall" problem, future computers are not based on computing memory, but the in-memory computing, thereby reducing the cost of data access in the calculation process.Figure 3. Conventional Computing vs In-memory Computing Ⅲ What Is In-memory Computing?3.1 In-memory Computing DefinitionIn-memory computing (or in-memory computation) is a technique based on RAM data storage and indexing, which proposed by the MIT research group, and its main purpose is to accelerate the convolution calculation. We know that convolution calculations can be expanded into weighted accumulation calculations. From another perspective, it is actually a weighted average of multiple numbers. Therefore, the circuit realizes the weighted average of the charge domain. The weight (1-bit) is stored in SRAM, and the input data (7-bit digital signal) becomes an analog signal through the DAC. According to the corresponding weight in the SRAM, the output is multiplied by 1 or -1 in the analog domain, which averaged in the analog domain, and finally read out by the ADC as a digital signal. Specifically, since the weight of the multiplication is 1-bit (1 or -1), it can be controlled by using a switch and a differential line simply. If the weight is 1, the capacitor on the side of the differential line is charged to the required output value. Otherwise, let the other side of the differential line be charged to this value. As for average, connect several differential lines together in the charge domain.Of course, there is more than one circuit for in-memory calculation, and the calculation accuracy is not limited to 1-bit. However, we can see the above examples that the core idea of in-memory calculations is generally to convert calculations into weighted calculations. Store the weights in the memory unit, then modifications on the core circuit of the memory (such as the readout circuit) are made. So that the process of reading is like a process in which the input data and weights are multiplied in the analog domain, that is, convolution. Because convolution is a core part of AI and other calculations, in-memory computing can be widely used in such applications. In-memory computing uses analog circuits for calculations, which is the difference compared with traditional digital logic calculations.In more traditional architectures, there are some multiply-accumulate circuits (MAC) for tensor math, especially the matrix multiplication. These architectures attempt to arrange the MAC in a way that moves weights and activations to the appropriate location. Activations are calculated from the previous neural network layer. Multiplication usually involves activations and weights, both must be moved to the place where multiplies them. In-memory computing makes use of it. Therefore, if the weights are stored in memory, the memory can access through activations to obtain multiplication and accumulation. The only difference from the actual memory is that the in-memory computing concatenates all word lines at once, instead of decoding the input to get one word line only.Figure 4. In-memory Computing Diagram3.2 Four Realization MethodsThe attempt is to enter the analog domain and treat the storage unit as an analog unit instead of a digital unit to reduce consumption. We have already got a way to use simulation on the front end of the inference engine. That is in-memory computing. Therefore, we take digital data, using a DAC to convert it to an analog value, and then driving a memory with these analog content to obtain an analog bit-line output, finally using an ADC to convert the result back to a digital format. However, the in-memory computing is still in the exploratory stage, and there are many specific implementation methods to study, currently there are three types: RRAM, Flash, SRAM, and DRAM.Based on RRAMRRAM is the most common method of doing this, because it is easy to use by applying Ohm's law to a series of resistors, but it still has the problem of relying on RRAM. The relationship between programming and resistance is non-linear, which requires more work to be done to make viable calculation circuits in RRAM memory for market. So it is just an idea, and the specific plan is still under study. Based on FlashNOR Flash memory has a more traditional word-line/bit-line structure. It is both resistive and capacitive. Generally, the memory cell is a transistor that is turned on or off. However, if it is partially conductive, it can be used as a resistor. The resistance depends on the amount of charge on the floating gate of the memory cell (capacitor). When running all the time, the cell will conduct to its maximum capacity. During this process, it does not conduct at all, however, it can be partially programmed. There is a problem is that you cannot precisely control the number of electrons. Moreover, the response to any number will vary with the process and temperature and other variables.Two companies are studying this method. Microchip owns their memBrain array, thanks to their acquisition of SST, and Mythic is a start-up company dedicated to an inference engine that uses in-memory computing with flash memory. Both companies said that they are using extensive calibration techniques to deal with this change.Another issue, flash cells will lose electrons over time. Electrons will flow around, which brings up an interesting topic: on this type of memory array, data retention and durability will be like.From the application point of view, it depends on whether it is to be used in cloud computing or edge inference engine. At the edge, it may perform certain fixed reasoning functions throughout the life cycle of the device. Therefore, if there are enough arrays, then you will load the weights for the first time and don't need to program it anymore (unless you do a update), because the flash memory is non-volatile. Although you still need to move activations, there is no need to move the weights, which will be stored permanently in the array. This would indicate that data durability (number of times the device can be programmed before cumulative damage accelerates electron leakages to an unacceptable level) does not matter, it only need to program once.In contrast, in cloud applications, the device is likely to be shared as a general-purpose computing resource, so this requires reprogramming for each new application. This means that battery life becomes more important in the cloud. Mythic claims to have a 10K write cycle, and has observed that even if it is reprogrammed every day, it will last for more than 10 years.If set an analog value for it and use an analog value in the cell, then in theory, each electron is important. However, if there is enough electron migration, you need to refresh the storage unit, or compensate for electrons change in some way. Because the same analog input today will produce different results than a year ago. The calibration circuit can also deal with some aging problems. However, for data retention, Mythic said they do perform regular updates of the weight values stored in flash memory. This will make persistence the main wear-out mechanism rather than data retention. Microchip stated that its data retention time is TBD, but it is likely to reprogram the device quarterly or annually to restore the unit.So they need a large number of high-quality ADCs and DACs to keep the signal-to-noise ratio (SNR) within a scope of accurate reasoning, which is the focus of designing work. Mythic claims that they provide a novel ADC, so that Microchip can share it to reduce the number required. Although ADC does consume energy, it also greatly reduces overall system consumption. Based on SRAMThis idea came from a lecture at Hot Chips at Princeton University. By definition, SRAM is a bistable unit. Therefore, it cannot be in an intermediate state, how should this be handled? And the DACs and ADCs that need to be corrected more over than the array in terms of area and power consumption.The point of this problem boils down to the question of how to simulate. They explained that this method uses more than one-bit line for calculation. Since the unit is still a digital value, it takes several bit lines to perform a calculation. The bit line can be split, and different groups perform different multiplications. The following figure illustrates it.Figure 5. Bit LineWith 8 inputs at a time, so the input vector is sliced and several consecutive multiplications are carried out to obtain the final results. The bit line charge is deposited on the capacitor. When ready to read, the charge is read out and sent to the ADC for conversion back to the digital domain. Their basic unit structure is as follows:Figure 6. Bit CellThese capacitors may affect chip size issues, but they said that the metal above the cell can be used. Of course, one cell is now 80% larger than the standard 6T SRAM cell (even without capacitors), but they say that their overall circuit is still much smaller than a required circuit based on standard digital implementation. In addition, since their basic array operations are still in digital form, they are less sensitive to noise and changes, which means their ADCs can be simpler and consume less power.Figure 7. Chip SizeBased on DRAMThis idea refers to not using a lot of power to obtain DRAM content, and in some way incorporate calculations into the CPU or other computing structures and directly run it on the DRAM die, which is what UPMEM does. A simple processor is built on the DRAM die, also the architecture will not compete with Xeon chips, they call this set "processing in memory" or PIM.Figure 8. PIM ChipInstead of bringing data to calculations, they bring calculations to data. The runtime is performed by the CPU in DRAM chip. That is, there is no need to move the data to any location outside of the DRAM chip, just send the calculating result back to the host system. Also, since ML calculations usually involve a lot of reduction, less data required for calculations. Although this does require some minor changes to the DRAM, they did not change the manufacturing process. Under this case, a standard DRAM module will provide multiple opportunities for distributed computing. At the same time, it becomes complicated to use this function to write a program.They said that a server using PIM offload will consume twice as much power than a standard server connected to a DRAM module without PIM. However, with a throughput of 20 times, it still provides them with a 10 times energy efficiency advantage. In addition, this method can help defend against side-channel security attacks. Thus a group of computing threads originally contained in one or more CPUs flows to DRAM. Therefore, it is necessary to check all DRAMs and figure out where thread is in some way, but this will be a difficult task. Ⅳ Driving Force of In-memory Computing and Market Prospects4.1 In-memory Computing for AIPeople have recognized the problem of "memory wall" for a long time, but why is in-memory computing only raised in the past two years? So we have to analyze the boost behind its rise.The first motivation is the rise of AI based on neural networks, especially the hope that AI can be popularized in mobile and embedded devices. So that in-memory computing with a high energy efficiency ratio has attracted attention. In addition, neural networks have a high tolerance for errors in calculation accuracy. Therefore, errors introduced in simulation calculations of in-memory computing can often be accepted. That is to say in-memory computing and AI are good partners for each other.The second motivation is the new memory. For in-memory computing, the memory characteristics often determine the efficiency of in-memory computing. Therefore, new memories improvement will often drive the development of in-memory computing. For example, the recently popular ReRAM uses resistance modulation to store data, so the readout of each bit uses a current signal instead of a traditional charge signal. In this way, it is a very natural operation for current to accumulate (combining several currents directly to achieve the sum of currents, even without additional circuits). That is to say, ReRAM is very suitable for in-memory calculations. From the perspective of memory promotion, new memories are also willing to catch up with the AT trend. Therefore, new memory manufacturers are also happy to see in-memory computing based on their own memories to accelerate AI development, which will broaden the memory market. 4.2 In-memory Computing Product OutlookChip products for in-memory computing are expected to come in two forms. The first form is sold as a memory IP with computing functions. Such memory IP may be traditional SRAM, or new memory such as eFlash, ReRAM, MRAM, and PCM.The second form is to directly build AI acceleration chips based on in-memory calculations. For example, Mythic plans to make PCIe accelerator cards based on flash memory, that is, access data with the main CPU through the PCIe interface. The weight data is stored on the Mythic memory chip, so that when the data is sent to the Mythic IPU, the calculation can be directly read out. In this way, the action of reading the weights data is eliminated.Figure 9. Mythic is a Pcie Accelerator 4.3 In-memory Computing Market and ProspectWhat impact will in-memory computing have on the AI chip market? First of all, we see that in-memory computing uses analog calculations, so its accuracy will be affected by the low signal-to-noise ratio. Usually the upper limit of accuracy is about 8-bit, and it can only do fixed-point calculations not the floating-point calculations. So in-memory computing is not suitable for the AI training market that requires high calculation accuracy. In other words, the main battlefield of in-memory computing is the AI inference market. For example, it is more suitable for embedded artificial intelligence, which has high requirements for energy efficiency not the accuracy. In fact, in-memory computing is actually most suitable for occasions where large memory is needed. For instance, flash is inherently required in IoT and other scenarios, so if you can add the in-memory computing to flash, it is quite suitable. However, introducing in-memory computing in a large storage memory may not appropriate. Based on this analysis, we believe that in-memory computing may become an important part of embedded AI (such as smart IoT) in the future. Ⅴ ConclusionWith the rise of AI and new memories, in-memory computing has also become a new hot spot. Based on the unique characteristics of the memory, it combines with analog calculations in memory, thereby greatly reducing the memory read and write operations in AI. Although the accuracy of calculation in the memory is limited by analog calculation, it is also suitable for embedded AI applications that pursue energy efficiency most and can accept a certain loss of accuracy. Frequently Asked Questions about In-Memory Computing Technology1. Why do we need in memory computing?In-Memory Computing provides super-fast performance (thousands of times faster) and scale of never-ending quantities of data, and simplifies access to increasing numbers of data sources. 2. What does in memory mean?An in-memory database is a type of purpose-built database that relies primarily on memory for data storage, in contrast to databases that store data on disk or SSDs. ... Because all data is stored and managed exclusively in main memory, it is at risk of being lost upon a process or server failure. 3. How does in memory computing work?In-memory computing means using a type of middleware software that allows one to store data in RAM, across a cluster of computers, and process it in parallel. Consider operational datasets typically stored in a centralized database which you can now store in “connected” RAM across multiple computers. 4. What is in memory computing in SAP HANA?An In-Memory database means all the data from source system is stored in a RAM memory. In a conventional Database system, all data is stored in hard disk. It provides faster access of data to multicore CPUs for information processing and analysis. 5. How is data stored in memory?Normally memory is described as a storage facility where data can be stored and retrieved by the use of an address. This is accurate but incomplete. A computer memory is a mechanism whereby if you supply it with an address it delivers up for you the data that you previously stored using that address. 6. What is in memory data processing?In-memory processing is the practice of taking action on data entirely in computer memory (e.g., in RAM). ... Since the storage appears as one big, single allocation of RAM, large data sets can be processed all at once, versus processing data sets that only fit into the RAM of a single computer. 7. What is in memory database processing and what advantages does it provide?The major advantage of systems using in-memory databases vs traditional database systems is: its performance speed. ... Source data is loaded into the system memory in a compressed and format. Therefore, in-memory processing reduces disk seek time for accessing data and streamlining the work involved in processing queries. 8. What is big data computing?Big data computing is an emerging data science paradigm of multi dimensional information mining for scientific discovery and business analytics over large scale infrastructure. ... Big data is characterized by 5V's such as volume, velocity, variety, veracity, and value.

IntroductionPCB exists in every electronic device. A fully functional PCB is mainly used to create connections between components, such as resistors, capacitors, inductors, diodes, transistors, integrated chips, etc. It is the carrier of the entire logic circuit. Sound PCB design can save production costs, and achieve good circuit performance and heat dissipation effect. PCB designs vary in complexity according to product needs. This article mainly talks about wiring, one of the basics of PCB design.PCB Design: From Idea to Schematic to PCBCatalogIntroductionⅠ PCB Basics: Wiring RulesⅡ Three PCB Wiring MethodsⅢ PCB Design: Wire InspectionⅣ Complete PCB Design Projects Inspection4.1 General PCB Design Inspection Projects4.2 PCB Electrical Characteristics Checking Projects4.3 PCB Physical Characteristics Checking Projects4.4 PCB Mechanical Design Factors4.5 PCB Installation Requirements4.6 PCB Pull-out Requirements4.7 PCB Mechanical Considerations4.8 PCB Electrical Considerations4.9 Electronics Inspection Before Into A PCBⅤ ConclusionⅠ PCB Basics: Wiring Rules1. The area within 1mm from the edge of the PCB board and within 1mm around the mounting hole will not take wiring.2. The power line width should not be less than 18mil; the signal line width should not be less than 12mil; the cpu input and output lines should not be less than 10mil (or 8mil); the line spacing should not be less than 10mil.3. It is necessary noted that the power line and the ground line should be as radial as possible, and the signal line must not be looped.4. Ground circuit rulesThe loop area formed by the signal line should be as small as possible. The smaller the loop area, the less external radiation and the less interference from the outside. An example is shown in the figure below:5. Crosstalk controlHere crosstalk refers to the mutual interference caused by long parallel wiring between different networks on the PCB, which caused by the distributed capacitance and inductance between the parallel lines. The main measures to overcome it are:a. Increase the spacing of parallel wiring and follow the 3W rule. To ensure that the distance between the lines is large enough, when the distance between the line and the center of the line is not less than 3 times the line width (as shown in the figure below). If the line center distance is not less than 3 times the line width, 70% of the line electric fields will not interfere with each other, which is called 3W rule.b. Insert a grounded isolation wire between the parallel wires. Reduce the distance between the wiring layer and the ground plane.6. The direction control rules of routing:The routing directions of adjacent layers are orthogonal. Different signal lines in the same direction on adjacent layers should be avoided to reduce unnecessary interlayer crosstalk. When the signal rate is high, use a ground plane to isolate each wiring layer, in other words, isolate each signal line with ground line. The neighbouring wires used in the input and output end of the circuit shouldn’t be parallel to prevent the feedback, and it is best to add a ground wire between these wires.7. Open loop inspection rules for wiring:Generally, it is not allowed to have a floating wiring at one end, because of the "antenna effect" and unnecessary interference radiation and reception, which may bring unpredictable results.8. Impedance matching inspection rulesThe wiring width of the same network should be kept the same. Line width variations will bring uneven line characteristic impedance, and reflection will occur when the transmission speed is high. This situation should be avoided in the design. Under certain conditions, such as the lead wires of the connector and the similar structure of the lead wires of the BGA package, the change of the line width may not be avoided, so that the length of the middle inconsistent part should be minimized.9. Wiring closed loop inspection rules:Prevent signal lines from forming self-loops between different layers. Such problems are prone to occur in multilayer board design, and it will cause radiation interference. As shown below:10. The branch length control rule of wiring:Try to control the length of branches, and the general requirement is Tdelay≤Trise/20.11. Resonance rules of wiring:For high-frequency signal design, the wiring length must not be an integer multiple of its wavelength to avoid resonance.12. Line length control rules:In fact, it refers to the short-circuit rule. When designing, you should keep the wiring length as short as possible to reduce interference problems caused by unnecessary lines. Especially for some important signal lines, such as clock lines, be sure to place oscillators close to the device. In the case of driving multiple devices, the network topology should be decided according to the specific situation.13. Parallel input and output wires on the PCB board should be avoided as far as possible to avoid parallel. It is best to place a ground wire between the two wires to avoid circuit feedback coupling.14. Digital ground and analog ground should be separated. For low-frequency circuits, single-point parallel grounding should be used. High-frequency circuits should be grounded in series with multiple points. For digital circuits, the ground wire should be closed into a loop to improve anti-noise capability.15. The wiring and via distribution of the whole circuit board should be uniformity. When the outer signal of the circuit board has a large blank area, auxiliary lines should be added to make the lines distribution on the board basically balanced.16. The low-frequency circuit can be grounded at a single point in parallel, and the actual wiring can be connected in series and then grounded in parallel. The high-frequency circuit can be grounded in series with multiple points. The ground wire should be short and thick. For high-frequency components, a large area ground foil can be used. The ground wire should be as thick as possible. If the ground wire is a very thin, the ground potential will change with the current, which reduces the noise resistance.17. Multilayer boards should be as symmetrical as possible when designing the laminated structure, as well as the wiring density and copper layout of each layer to reduce warpage and reduce EMI during soldering.18. The signal line should not cross the power supply and ground. The signal reference plane should be as complete as possible.19. Impedance controlThe signal lines that need impedance control must be wired in strict accordance with the calculated data, in addition, it is necessary to tell manufacturers it. For signal lines that do not require it, the impedance should be calculated to prevent unnecessary interference.20. Grid copper should be used less in low frequency circuits. Although it can effectively reduce the problem of large area copper skin blistering. When using grid copper, you need to consider the electrical length of the grid line and the working frequency of the circuit board. If using grid copper, the power supply should also be coated with solid copper as much as possible.21. A group of buses with the same attribute should be wired side by side as much as possible, and the length should be as equal as possible. Ⅱ Three PCB Wiring MethodsThe wires should take the shortest route between components according to the specified wiring rules. Limit the coupling between parallel wires as much as possible. Good PCB design requires the minimum number of wiring layers, and also requires fair use of the widest wire and the largest pad size corresponding to packaging density. For example, rounded corners and smooth inner corners design may avoid some electrical and mechanical problems, therefore, sharp corners and sharp corners in the wire should be avoided. Here introduces three main PCB routing methods; right-angle wiring, differential wiring, and serpentine wiring to illustrate PCB layout:A. The influence of right-angle wiring on the signal is mainly reflected in three aspects:1. The corner can be equivalent to the capacitive load on the transmission line to slow down the rise time.2. Discontinuous impedance will cause signal reflection.3. The EMI generated by the right-angle tip reaches the RF field above 10GHz. Such a right-angle is likely to develop into the source of high-speed problems. B. To figure out what is differential wiring, you must first understand what is differential signal. In a word, the driving end sends two equal and inverted signals, and the receiving end judges the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces. Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:1. Have Strong anti-interference ability. Because the coupling between the two differential traces occurs, when there is noise interference from the outside, they are almost coupled to the two lines at the same time. However, the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely canceled.2. It can effectively suppress EMI. Due to the opposite polarity of the two signals, the electromagnetic fields radiated by them can cancel each other out. What’s more, the tighter the coupling, the less the electromagnetic energy leaked to the outside world.3. The timing positioning is accurate. Because the switch change of the differential signal is located at the intersection of the two signals. Unlike ordinary single-ended signals, which rely on the high and low threshold voltages to judge. Timing positioning is less affected by the process and temperature, and also more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signaling technology. C. Serpentine line is a type of wiring method often used in PCB layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when a signal is transmitted on a serpentine trace, the parallel line segments will be coupled in a differential mode. The smaller the S, the greater the Lp, the greater the coupling. It may cause the transmission delay to be reduced, also the signal quality is greatly reduced due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. The following are some suggestions when dealing with serpentine wring:1. Try to increase the distance (S) of parallel lines, at least more than 3H(H refers to the distance from the signal trace to the reference plane). As long as S is large enough, the mutual coupling effect can be almost completely avoided.2. Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.3. The signal transmission delay caused by the strip-line or embedded micro-strip line is less than that of the micro-strip. Theoretically, the strip-line will not affect the transmission rate due to differential mode crosstalk.4. For signal lines with high-speed and strict timing requirements, try not to take serpentine lines, especially in a small area.5. You can often use s-shaped routing at any angle, which can effectively reduce the mutual coupling.6. In high speed, the serpentine line has no ability so-called filtering or anti-interference, and can only reduce the signal quality, so it is better to use for timing matching.7. Sometimes you can consider the spiral routing method for winding. Simulation shows that its effect is better than normal serpentine routing.Ⅲ PCB Design: Wire Inspection1.Wire SpacingThe minimum spacing of wires must be determined to eliminate voltage breakdown or arcing between adjacent wires. The spacing is variable, it mainly depends on the following factors:1) Peak voltage between adjacent wires2) Atmospheric pressure (maximum working altitude)3) Coating layer4) Capacitive coupling parametersComponents with critical impedance or high-frequency components should be placed very close to reduce the critical stage delay. There is something need to pay attention to. Transformers and inductive components should be isolated to prevent coupling. Inductive signal wires should be laid orthogonally at right angles. Components that generate any electrical noise due to magnetic field movement should be isolated or rigidly installed to prevent excessive vibration.2. Whether the wire is short and straight without sacrificing function.3. Whether the restrictions on the wire width are complied with.4. There must be a minimum distance between wires, wires and mounting holes, wires and pads.5. Whether to avoid all the wires (including component leads) closer to parallel wiring.6. Whether sharp corners (≤90℃) are avoided in the wire pattern. Ⅳ Complete PCB Design Projects Inspection4.1 General PCB Design Inspection Projects1) Has the circuit been analyzed? Is the circuit divided into basic units to smooth the signal?2) Does the circuit allow short or isolated key leads?3) Where must be shielded, are they effectively shielded?4) Have you made full use of the basic grid graphics?5) Is the best size of the printed circuit board?6) Do you use the available wire width and spacing as much as possible?7) Has the preferred pad size and hole size been used?8) Are the base plate and the sketch consistent?9) Is less cross-wiring used? Do cross wires pass through components and accessories?10) Are the letters visible after assembly? Are their size and model correct?11) In order to prevent blistering, is there any window on the large area of copper foil?12) Are there tool positioning holes?4.2 PCB Electrical Characteristics Checking Projects1) Have you analyzed the influence of wire resistance, inductance, and capacitance, as well as the critical voltage drop on the ground?2) Does the wire spacing and shape meet the insulation requirements?3) Has the insulation resistance value been controlled and specified in key areas?4) Is the polarity fully recognized?5) According to geometric view, has the effect of wire spacing on leakage resistance and voltage been measured?6) Has the medium for changing the surface coating been identified?4.3 PCB Physical Characteristics Checking Projects1) Are all pads and their positions suitable for final assembly?2) Can the assembled PCB meet the shock and vibration conditions?3) What is the required spacing of standard components?4) Are the components that are not firmly installed or the heavier parts fixed?5) Is the heating element heat dissipation and cooling normally? Or is it isolated from the printed circuit board and other heat-sensitive elements?6) Are the voltage divider and other multi-lead components placed correctly?7) Is the arrangement and orientation of components easy to check?8) Has it eliminated all possible interference on the printed circuit board?9) Is the size of the positioning hole correct?10) Are the tolerances complete and reasonable?11) Have you controlled and signed the physical properties of all coatings?12) Is the ratio of via hole and lead diameter within an acceptable range?4.4 PCB Mechanical Design FactorsThe printed circuit board adopts mechanical methods to support the components, however, it cannot be used as an unique structural part of the entire device. On the edge of the printing plate, at least every 5 inches for a certain support. The factors that must be considered when selecting and designing printed circuit boards are as follows:1) The size and shape of the printed circuit board.2) The type of mechanical accessories and plug (seat) required.3) The environmental adaptability of circuits.4) According to some factors, such as heat and dust, install the printed circuit board vertically or horizontally.5) Some environmental factors that require special attention, such as heat dissipation, ventilation, shock, vibration, and humidity, dust, and radiation, etc.6) Physical support7) Install and fix.8) Disassemble4.5 PCB Installation RequirementsAccording to practical experience, the distance between the supporting points of a printed circuit board with a thickness of 0.031-0.062 inches should be at least 4 inches. For a printed circuit board with a thickness greater than 0.093 inches, the distance between the supporting points should be at least 5 inches. Taking this measure can improve the rigidity of the printed circuit board and avoid possible resonance. The following factors should be considered before deciding which mounting technology they use.1) PCB structure.2) Input and output terminals.3) Available equipment space.4) Convenience of loading and unloading.5) Type of attachments.6) Required heat dissipation.7) Required shieldability.8) The type of circuit and its relationship with other circuits.4.6 PCB Pull-out Requirements1) The influence of plugging tools on the installation distance between two printed circuit boards.2) When the plug-in tool used in the equipment, its size should be considered.3) A plug-in device is required, which is usually fixed to the printed circuit board assembly with rivets.4) As for the mounting frame of the printed circuit board, special design such as load bearing flange is required.5) The adaptability of the plug-in tool used and the size, shape and thickness of the printed circuit board.4.7 PCB Mechanical ConsiderationsThe characteristics of the board substrate that have an important influence on the printed circuit assembly are: water absorption, thermal expansion coefficient, heat resistance, flexural strength, impact strength, tensile strength, shear strength and hardness. All these characteristics affect the function and the production efficiency of the printed circuit board structure. For most applications, the dielectric substrate materials of the printed circuit board are as following:1) Phenolic impregnated paper2) Acrylic-polyester impregnated randomly arranged glass mat3) Epoxy impregnated paper4) Epoxy impregnated glass clothEach substrate can be flame retardant or combustible. The first 3 types mentioned above can be processed. The most common used material for printed circuit boards with metalized holes is epoxy-glass cloth. Its dimensional stability is suitable for high-density circuits and can minimize the occurrence of cracks in the metalized holes. One disadvantage of epoxy-glass cloth laminate is that it is difficult to punch in the usual thickness range of printed circuit boards. For this, all holes are usually drilled and copied and milled to form a print shape of the circuit board.4.8 PCB Electrical ConsiderationsIn DC or low-frequency AC applications, the most important electrical characteristics of insulating substrates are: insulation resistance, anti-isolation, printed wire resistance, and breakdown strength. In high frequency and microwave applications, include: dielectric constant, capacitance, and dissipation factors. In all applications, the current carrying capacity of printed wires is important.4.9 Electronics Inspection Before Into A PCB1) Check the rationality and correctness of the schematic diagram.2) Check the correctness of the component packaging of the schematic.3) The distance between strong and weak current lines, and the distance between isolation areas.4) Check the schematic diagram and PCB diagram to prevent the loss of the network table.5) Whether the package of the component matches the physical object.6) Whether the placement of the components is appropriate.7) Whether the components are easy to install and disassemble.8) Whether the temperature sensitive element is too close to the heating element.9) Whether the distance and direction of the mutual inductance components are appropriate.10) Whether the placement between the connectors is smooth.11) Easy to plug in and plug out12) Input and output13) Strong current and weak current14) digital and analog should be interlaced.15) Arrangement of elements on the upside and downside16) Check whether the directional component has been wrong flipped instead of rotated.17) Check whether the mounting holes of the component pins are suitable and whether it is easy to insert.18) Check whether the empty pin of each component is normal and whether it is a missing line.19) Check whether there are vias between the upper and lower wiring of the same net table. And the pads are connected through the holes, to prevent disconnection and ensure the integrity of the circuit.20) Silk screen printing should be clear, so that the operation of welding or maintenance can be easy.21) The arrangement of power and signal lines in the socket should ensure signal integrity and anti-interference.22) Pay attention to the proper ratio of pads and solder holes.23) Each plug should be placed on the edge of the PCB board as much as possible and easy to operate.24) Whether the size and distribution of the mounting holes on the PCB are appropriate to reduce the PCB bending stress.25) Pay attention to the height distribution of the components on the PCB to ensure easy assembly.Ⅴ ConclusionBased on the above mentioned rules, drawing the PCB schematics you need becomes easier. Decide what PCB you want to and install a PCB design software. PCB software is really helpful and powerful. Also a software can check your design to make sure the design does not contain errors such as traces that incorrectly touch, traces too skinny, or drill holes that are too small. For example, run the Electrical Rules Checker (ERC) to see if you’ve made any typical errors. There is less thing stopping you from making your first PCB, right? Frequently Asked Questions about PCB Design Diagram1. Which side of PCB is correct for soldering?The bottom side of the PCB is usually the side without components and the side that touches the solder wave during assembly. That is why sometimes it is also called SOLDER side. However more often, PCB are populated on both sides and the assembly process does not require wave soldering. 2. Which soldering method is suitable for soldering printed circuit board?Soldering Iron – Used to melt solder and connect component pins to board pads. A cheap soldering pencil may be sufficient, but a temperature-controlled solder station is best for high performance boards. Solder – An alloy of tin and lead with a low melting point. 3. What is PCB diagram?A PCB schematic is a simple two-dimensional circuit design showing the functionality and connectivity between different components. ... Once the blueprint has been completed, the PCB design comes next. The design is the layout, or physical representation of the PCB schematic and includes the copper track and hole layout. 4. Why we use PCB in soldering?PCB soldering is another term for the process of soldering electrical circuit boards. ... As the soldering iron melts this metal, it is then used a bit like glue to stick to pieces together. As the solder metal cools, it will re-harden into one large shape that connects the two parts. 5. How do you read a PCB board?Start with an easy analog circuit, such as a guitar distortion pedal, and work your way up to more complicated versions. Make a drawing of the top of the circuit board. Show the positions of the capacitors, integrated circuits, resistors, transistors and other components. Review it to make sure everything is included.

Ⅰ IntroductionComputer memory is used to store programs and data. The main function of memory is to read and write. For random access memory RAM, their main functions are to read and write, and for read-only memory ROM, the main function is only to read. In general, the memory can be a card,  a floppy disk, etc., they can be active or fixed, which used to access data.This Video Introduce How Memory Store Data and How CPU Access Them.Program is the basis of computer operation, and data is the object of computer operation. Regardless of whether it is a program or data, it is expressed in binary form in the memory, and is collectively referred to as information. In a computer, the memory capacity represents by byte (abbreviated as B) as the basic unit, a byte is composed of 8 binary bits. In addition to bytes, the storage capacity is expressed in KB, MB, GB, and TB (which may be referred to as K, M, G, and T, respectively. For example, 128MB may be referred to as 128M). Among them, 1KB=1024B, 1MB=1024KB, 1GB=1024MB, 1TB=1024GB.CatalogⅠ IntroductionⅡ How to Store and Access Data?Ⅲ What Computer Memory Do?3.1 Explain Computer Memory3.2 Binary Decoder3.3 Chip Select & BusⅣ Example: 8086 MicroprocessorⅤ Technology Improvement5.1 What is Direct Memory Access (DMA)?5.2 DMA Transfer Modes5.3 DMA Transfer Process5.4 DMA Advantages and DisadvantagesⅥ Questions Related to Computer Memory WorksⅡ How to Store and Access Data?Before you know how the electronic memory works, it is necessary to get a general idea of the normal computer operation.Turn on the computer.First, the computer loads data from ROM and executes a power-on self-test (POST) to ensure that all major components are working properly. As part of this test, fast read/write operations check all memory addresses to ensure that there are no errors in the memory chip by memory controller. Read-write operation means writing data to a certain bit and then reading from it.Second, the computer loads the basic input/output system (BIOS) from the ROM. The BIOS provides the most basic information about storage devices, boot order, security, automatic identification functions, and other basic items.The computer loads the operating system from the hard drive into the system RAM. Normally, as long as the computer is turned on, the critical part of the operating system is kept in RAM. This allows the CPU to immediately access the operating system, thereby enhancing the performance and functionality of the entire system.Third, when an application is opened, it will be loaded into RAM. To save RAM space, only the basic part of the app programs are loaded, and then load other parts as needed. After the application is loaded, all files that have been opened for use in the RAM. When saving files and closing applications, files will be written to the designated storage device, and then the loading will be cleared from the RAM. It should be noted a fact that, if the changed files are not saved to the permanent storage device before being erased, they will be lost.In the above operation, every time the content is loaded or opened, it is put into RAM. This only means that it has been put into the temporary storage area of computer so that the CPU can more easily access the information. The CPU requests the required data from the RAM, processes it, and then writes the new data back to the RAM with successive cycles. In most computers, data processing reached millions of times between the CPU and RAM per second.Ⅲ What Computer Memory Do?3.1 Explain Computer MemoryThe memory is composed of a storage body, an address decoder, a read-write control circuit, an address bus, and a data bus.A semiconductor memory is like a small drawer, and there are eight small grids in it. Each small grid is used to store charge. The charge is transferred in or discharged through the wire connected to it. It is easy to understand, if you think of a wire as a water pipe, and the electric charge in the small grid is like the water. Each small drawer in the memory is a place to store data, which we call a cell.Figure 1. Computer MemoryThere are many cells in a memory, and the lines are connected in parallel. When the charge is applied, the charge will fill all the cells. When the charge is released, the charge in each cell will be discharged. This is of course not what we want. To avoid it, the memory structure should be changed slightly. There is a control line on each cell. Which unit you want to put the data, a signal is given to the control line of this unit. Be specific, this control line is like a switch, the charge can flow freely when switch on, and there is no signal on the control line of other cells to affect each other. So as long as you control the control lines of different cells, you can write different data to each unit. In the same way, if you want to get data from a unit, you only need to turn on the corresponding control line. 3.2 Binary DecoderFigure 2. DecoderFirst, how to control the control lines of each unit is not easy. For example, there are 655,36 units in a 27,512 memory chip, and each wire has to led out. This integrated circuit must have more than 60,000 pins. Obviously, this is clumsy. At this time, a way of decoding is made. Let’s briefly introduce it: a line can represent 2 states, 2 lines can represent 4 states, etc. And so on, 16 lines can be represented for 65536 states. 3.3 Chip Select & BusNext to the last question, let us focus on another problem. Where did the eight wires connect with each cell come from? In general, there are connected from the computer, and they also connect with other parts except for a memory chip. In this case, a problem arises. Since these eight lines are not dedicated between the memory and the computer, if you always connect a unit to these eight lines, this may cause confusion . For example, the value in a memory unit is 0FFH, in other memory cell is 00H, it is hard to figure out the high level or low level which these lines represent. So we have to separate them. The method is, when external wires are connected to the pins of the integrated circuit, they are not directly connected to each unit, and a group of switches is added between chip and computer. If we really want to write data to this memory or read data from the memory, then just turn on the switch. This group of switches is selected by three leads: read control end, write control end and chip select end.To write data to the chip, first is selecting chip, then send out a write signal, and the switch is turn on, therefore, the data is written to the chip. If you want to read, first is selecting chip, then send the read signal, and the switch is closed, the data is sent out. In addition, the read and write signals should be connected to another memory. Since the chip select terminals are different, there is no conflict when having read or write operation. Many people still have a question, will these two chips be selected at the same time? As long as it is a well-designed system, it will not happen, because it is controlled by mathematical calculation, not a manual control. If there is a situation where two chips are selected at the same time, it may be a circuit failure.It can be seen from the above that the eight lines used to transfer data are not dedicated, but are shared by many components, so we call them the data bus. The other 16 address lines are also connected together, called the address bus. Ⅳ Example: 8086 MicroprocessorThe CPU is connected to the storage unit and the I/O interface circuit through an address bus, a data bus, and a control bus. So how does the CPU access a certain address in the memory? Next, we will use the 8086 microprocessor architecture as an example, because its structure is simple and easy to introduce.Figure 3. 8086 MicroprocessorIf the CPU wants to read and write memory data, there must be wires to connect them together. In a computer, this kind of wire is called a bus. If you disassemble the computer case, it is easy to see that there is a collection of wires. These wires send signals at the same time, and each wire is either high or low level. The bus is divided into address bus, data bus and control bus according to different functions. Taking the above diagram as an example, the CPU needs to read the information at address 3, which is roughly divided into the following steps:The CPU outputs the physical address 3 to the address line.The control line needs to select the corresponding storage device, and then inform the device that data will be read from it.The storage device will send data 8 to the data line.Figure 4. 8086 CPUFrom here we see that there is a bus connection between the CPU and the storage device. In fact, there is a bus inside the CPU. It connects different components, such as registers, operators, and controllers. But in the computer, the bit number of different buses is not necessarily the same. For example, the internal bus of the 8086 is 16-bit, the address bus is 20-bit, and the data bus is 16-bit.Another question, since the 8086 CPU is a 16-bit structure, how can it output a 20-bit physical address? In fact, it is very simple. The address adder solves this way: segment address (16 bits) x 16 + offset address (16 bits) = physical address (20 bits), such as B800H x 16 + 1111H = B9000H. If you are not familiar with hexadecimal, then we can use decimal to describe this calculation. For example, home, school, and library are on a straight line. The distance between them is as shown in the following figure. Show the distance of the library: the library is 200m away from home, which is actually its physical address. But now there are some limits. We can only communicate with our friends through paper slips. Unfortunately, we can only write 2 digits on the papers, and the number of paper is not limited, so we agreed on the rule: paper 1x10+paper 2 = physical address, for example, write 11 on the paper 1 and 90 on the paper 2, which means that the school is 110m away from home and the library is 90m away from the school. The maximum distance that these two pieces of paper can represent is 99x10+99=1089.The above mentioned is the concept of offset address of the segment address. Let's imagine that if the internal bus of the 8086 CPU is 20-bits, it can directly represent the physical address. Therefore, the concept of segment address is not critical here. What's more, the CPU hardware design will change in the future, and it may be completely different.What we should know is how the CPU reads information from memory, and how does the CPU know whether the read information is ordinary data or a program that needs to be executed? We know that there are many registers in the CPU (that is used to store information), it specifies 2 registers, called CS, IP (CS is used to store the segment address, IP is used to store the offset address). They represent the physical address of the current machine code that needs to be executed. During the execution of the code, the CPU will maintain the values of CS and IP. For example, for each execution of the machine code, IP will increase the corresponding value to point to the next instruction. By analogy, we can use other registers to represent the physical address of the data. Therefore, the binary information in the memory has no difference to the CPU. Distinguishing the program and the data depends on the register. Ⅴ Technology ImprovementWhether it is a PC card or the high-speed read-write disk in the storage system, we can not do data operation without the support of a hardware DMA.Figure 5. 8237 DMA Controller5.1 What is Direct Memory Access (DMA)?DMA refers to the interface technology that the external device directly exchanges data with the system memory without going through the CPU. It is a high-speed data transfer method that allows direct reading and writing of data between external devices and memory, neither through the CPU nor CPU intervention.To read the data of the peripherals into the memory or transfer the data of the memory to the peripherals, it is generally done through CPU control, such as CPU program query or interrupt mode. Using interrupts for data transfer can greatly increase the CPU utilization. But it has shortcomings. For a high-speed I/O device and the case of batch exchange of data, the DMA method can be used to solve the efficiency and speed problems. DMA directly exchanges data between peripherals and memory, so the speed of data transfer depends on the working speed of the memory and peripherals.The data transfer operation is performed under a "DMA controller". In addition to the CPU doing a little processing at the beginning and end, the CPU can execute other tasks during the transfer. In this way, the CPU and I/O are in parallel operation. Therefore, the efficiency of the computer system is greatly improved.Figure 6. DMA ControllerWhen realizing DMA transmission, the bus is directly controlled by the DMA controller. Therefore, there is a problem of bus control transfer. That is, before the DMA transfer, the CPU should hand over the bus control to the DMA controller, and after the transfer is done, the DMA controller should immediately return the bus control to the CPU. 5.2 DMA Transfer ModesPeripherals can directly access the memory through the DMA controller, and at the same time, the CPU can continue to execute programs. So how does the DMA controller and CPU use memory in the same time? The following three modes are generally used:(1) Burst mode: Stop the CPU access.(2) Cycle stealing mode: DMA return the control of buses to CPU after transfer of one word at a time.(3) Transparent mode: DMA and CPU access memory alternately. Burst ModeWhen the peripheral device requests to transfer a batch of data, the DMA controller sends a stop signal to the CPU, requesting the CPU to give up to use the address bus, data bus, and related control bus. After the DMA controller obtains the bus control right, it starts the data transfer. After a batch of data has been transferred, the DMA controller informs the CPU that it can use the memory and returns the bus control to it. Figure (a) is a time chart of this transmission method. Obviously, in this DMA transfer process, the CPU is basically in a non-working state or stands by.Advantage: It is suitable for group transmission of equipment requiring high data transmission rate.Disadvantages: In the internal access stage of the DMA controller, the memory performance is not fully utilized, that is to say, a considerable part of the memory work cycle is idle. This is because the interval between two data transmitted by a peripheral device is generally always greater than the memory storage period, even for high-speed I/O devices. For example, a floppy disk requires about 32us to read an 8-bit binary number, and the storage period of semiconductor memory is less than 0.5us, so many idle storage periods cannot be used by the CPU. Cycle Stealing ModeWhen the I/O device has no DMA request, the CPU accesses the memory as required by the program. Once the I/O device executes a DMA request, one or several memory cycles will be embezzled.The time sharing of this transmission mode is as follows:　1) At this time, the CPU does not need to access RAM, for example, the CPU is executing a multiplication instruction. Due to the long execution time of this instruction, there is no conflict between the I/O access and the CPU access, that is, the I/O device stealing one or two memory cycles has no effect on the CPU execution.2) When the I/O device accesses, so does the CPU, which creates an access violation. In this case, the I/O device takes first, because it has an access time requirement, the previous I /O data must be accessed before the next request arrives. Obviously, the I/O device steals one or two memory cycles, which means that the CPU has delayed the execution of the instruction. More specifically, inserting a DMA request during the CPU's execution of the in-access instruction embezzles one or two memory cycles. Compared with the method of stopping CPU access, cycle stealing not only achieves I/O transfer, but also utilizes the efficiency of memory and CPU greatly. It is a win-win  method. However, I/O device diversion has the process of applying for bus control, establishing line control, and returning bus control. Transferring a word takes one cycle for RAM, but it is generally 2-5 memory cycles for DMA controllers (depending on the delay of the logic line). Therefore, the method is suitable for the case where the read/write cycle of the I/O device is greater than the RAM storage cycle. Transparent ModeIf the CPU's work cycle is much longer than the memory access cycle, this method can make the highest efficiency of CPU and DMA access at the same time. Assuming that the CPU work cycle is 1.2μs and the memory access cycle is less than 0.6μs, then a CPU cycle can be divided into two sub-cycles, C1 and C2, where C1 is for DMA controller access and C2 is for CPU access.The time sharing of this transmission method is as follows: The following figure is the detailed time of DMA and CPU alternate accesses. C1 is dedicated to the DMA controller and C2 is dedicated to the CPU. This method does not require the bus usage right. It is allocated through C1 and C2. The CPU and the DMA controller each have their own control registers such as internal address registers, data registers, and read/write signals. In the C1 cycle, if the DMA controller has an access request, it can send signals such as address and data to the bus. In the C2 cycle, if the CPU has an access request, it also do the same process. In fact, for the bus, this is a multiplexer controlled by C1 and C2. This transfer of bus control power takes almost no time, so the efficiency of DMA transfer is very high.It is like transparent glass to the CPU, without any influence. Working in a transparent DMA mode, the CPU neither stops the main program running nor enters the stand-by state. It is an efficient working method, and the corresponding hardware logic is more complicated. 5.3 DMA Transfer ProcessFigure 7. DMA Working ProcessRequestThe CPU initializes the DMA controller and gives an operation command to the I/O interface, then the I/O interface issues a DMA request. ResponseThe DMA controller determines the priority and shielding of the DMA request, and makes a bus request to the bus adjudication logic. When the CPU executes the current bus cycle, the bus control can be released. At this time, the bus arbitration logic outputs a bus response, indicating that the DMA has responded, and notifies the I/O interface to take the DMA transfer through controller. TransferAfter the DMA controller obtains the bus control right, the CPU immediately stops s or only performs internal operations. The DMA controller outputs read and write commands to control the RAM and I/O interface directly. Under the control of the DMA controller, the data is directly transferred between the memory and the external device. In addition, it is necessary to provide the starting position and length of the data to be transferred. Rising an InterruptWhen the specified batch of data transfer is finished, the DMA controller releases the bus control right and sends an end signal to the I/O interface. When the I/O interface receives it, on the one hand, it stops the I/O device, on the other hand, it makes an interrupt request to the CPU. The CPU is free from the state of non-intervention, and performs a section to check the correctness of the DMA transfer operation code. Finally, the CPU will show the transfer result and carry out the original program.It can be seen that the DMA transfer method does not require the CPU to directly control the transfer, nor does it have the process of retaining and restoring the scene when having the interrupt process. Through the hardware, a direct path for data transfer is opened for the RAM and I/O devices, that is DMA. 5.4 DMA Advantages and DisadvantagesA:DMA reduces the clock cycle requires to read or write a patch of data, which improve the system operation efficiency.D:As a hardware device, running DMA control will increase cost.DMA can cause cache coherence problem. Ⅵ Questions Related to Computer Memory Works1. What is the purpose of computer memory?Computer random access memory (RAM) is one of the most important components in determining your system's performance. RAM gives applications a place to store and access data on a short-term basis. It stores the information your computer is actively using so that it can be accessed quickly. 2. What is the role of memory in a computer system?Computer memory or random access memory (RAM) is your system's short-term data storage; it stores the information your computer is actively using so that it can be accessed quickly. The more programs your system is running, the more memory you'll need. 3. Where is 8086 microprocessor used for?8086 Microprocessor is an enhanced version of 8085Microprocessor that was designed by Intel in 1976. It is a 16-bit Microprocessor having 20 address lines and16 data lines that provides up to 1MB storage. It consists of powerful instruction set, which provides operations like multiplication and division easily. 4. How does direct memory access DMA work?With DMA, the CPU first initiates the transfer, then it does other operations while the transfer is in progress, and it finally receives an interrupt from the DMA controller (DMAC) when the operation is done. ... DMA can also be used for "memory to memory" copying or moving of data within memory. 5. Why is DMA faster than CPU?The direct memory access or DMA mode of data transfer is faster amongst all the mode of data transfer . ... The device request the cpu through a DMA controller to hold its data ,address and control bus so that the device may transfer data directly to or from memory.

Ⅰ IntroductionIn recent years, with the rapid development of the internet of things (IoT) and electronic technologies, embedded devices such as mobile phones, smartwatches, and sports bracelets have become important elements of cloud computing, IoT, and big data analytics. Embedded terminal devices become more usual in daily life. However, to meet the high storage requirements of these increasingly diverse applications, scientific research personnel face more challenges. Non-volatile memory is more and more popular in the market due to its advantages such as low energy consumption, non-volatile, high density, and low latency. The following is a basic introduction to new non-volatile memory with good future development prospects.The Future of Non-volatile MemoryCatalogⅠ IntroductionⅡ TerminologyⅢ Concept and ClassificationⅣ One Question Related to Non-volatile Memory and Going Further4.1 Question4.2 AnswerⅡ TerminologyMemory is an important part of computer systems. According to different positions in the storage system, memory can be simply divided into three types: on-chip memory, main memory, and hard disk. Correspondingly, static memory (SRAM), dynamic memory (DRAM), and magnetic disks have become the traditional technologies to realize these storage system. Over the past 40 years, these three technologies have achieved tremendous technical and commercial success. With the exception of magnetic disks, the manufacture of both static and dynamic memory relies on semiconductor integration technology. Although these two technologies are different, they also have a common characteristic: they both use the accumulation and release of charge on semiconductor devices to achieve data storage. For example, in a dynamic memory, the charge is used to represent a logic 1, and the discharge represents logic 0. In static memory, this process is achieved by charging and discharging the parasitic capacitance of the transistor. For example, solid-state storage, which widely used in flash memory, also stores data by capturing and releasing the charge on the floating gate of the transistor.With the improvement of manufacturing technologies, the size of semiconductor nano devices has continued to shrink, and the charges that can be stored on all of the above traditional memory devices has also decreased, which has brought serious reliability issues: First, more leakage current; second, a small perturbation of charges will have a relatively greater impact. In addition, the inherent limitations of the charge storage mechanism itself also can't be avoided, the processing of traditional mainstream memories at the nanoscale and the process disturbances are also very challenging.From the aspect of functions, static memory and dynamic memory both belong to the volatile memory category. Its characteristic is that when the power supply is off, the data stored in the memory will disappear and cannot be recovered. Especially in the design of dynamic memory, the charge on the capacitor will gradually leak out as the time increases. Therefore, the stored info need to be refreshed periodically. The static memory doesn't have this issue, but the area of the memory cell is usually large (about 20 times that of the dynamic memory cell), causing serious leakage current. Flash memory belongs to the non-volatile memory, and the data stored on it will be retained for a long time after the power off. In terms of performance, the first two types of memory read & write speed are on the level of nanoseconds, while the flash memory are on the level of hundreds of microseconds or even milliseconds. For the reading mode, the static memory and the dynamic memory can achieve random storage, for example, any one or several memory cells can be read and written at will. It different for Flash, although its storage density is high, the read operation must be performed in units of pages. Moreover, the content update of the flash memory cannot be achieved by directly overwriting the original content, but must be written to a new erased page. In addition, the maximum times of read and write supported by flash memory is extremely limited, typically between thousands of and millions of times. Ⅲ Concept and ClassificationConcept: Non-volatile memory refers to the computer memory of the person whose stored data will not disappear when the power is turned off. It is characterized by non-volatile, byte-by-byte access, high storage density, low energy consumption, and fast read and write speed, but the read speed  far faster than write, in other words, they are asymmetric in a limited life.Classification: According to whether the data in the memory can be rewritten at any time, the traditional non-volatile memory can be divided into two categories: read-only memory (ROM) and Flash memory. New type non-volatile memory compared with traditional non-volatile memory, its device has greatly improved energy consumption, read and write speed, integration density, etc. At present, the newly developed new non-volatile memory mainly includes four types: dielectric memory (FRAM), magnetic medium memory (MRAM), Ovonic phase change memory (OUM), and polymer memory (PFRAM).The following highlights four new non-volatile memories.The technical limitations of traditional memories and the huge challenges brought by the light weight have prompted researchers to look for a new generation of memory devices. People want to find a memory with the following characteristics:1) Nano level read and write speed of static memory2) Integrated density with dynamic memory and even flash memory level3) Flash-like non-volatile memory featuresAlthough such a storage technology has not been fully realized at present, some very promising new storage devices have been developed, and some have even entered the production stage. The four newly developed non-volatile memories are very promising for data processing, because of the limitations of traditional non-volatile memories, it is very likely to replace flash memory in the future.Table 1. New Non-volatile Memories Basic SituationsItemPerformanceManufactureLimitationApplicationFerroelectric RAM(FRAM)1) low energy consumption 2) high read & write speed3) long storage time4) low power operation5) anti-radiationsmaller size1) limit read & write times2) low storage density3) low reliability1) RF IC card2) fast-start memory3) embedded cache4) aerospaceMagnetic RAM(MRAM)1) high read & write speed 2) unlimited read & write times3) low power operationsimple circuithigh cost1) storage2) industrial automation, 3) games 4) energy management 5) communication 6) transportation7) aerospace electronics8) sensor terminalsPhase Change Memory (OUM) 1) long service life2) safety 3) low energy consumption1) small capacity2) low cost1) lower read &write speed2) poor temperature characteristic1) wired and wireless communications2) consumer electronics 3) PC and embedded devicesPolymeric RAM1) good stability 2) low power operation1) 3D stack2) easy processing3) low cost1) limit read & write times, 2) destructiveness1) PC 2) digital camera 3) mobile phoneFRAMa. IntoductionFRAM is the non-volatile memory technology in the new generation. In terms of performance, it consumes low energy and can store data for a long time although there is power failure. It combines the characteristics of high read-write speed of RAM and long-term data storage of ROM. Embedded FRAM in the non-volatile memory situation of radiation-resistant and low-power has great significance. It can be embedded in the chip in a more direct way and has better performance than any other alternative chips. In terms of manufacturing and technology, FRAM is easier to reduce size than flash memory due to the advanced nodes (65 nm or smaller), and does not require the use of very thin oxides or high voltages. b. LimitationWhen FRAM reaches a certain times of read and write, FRAM cells will lose their durability, and the FRAM yield problem caused by array size restrictions and further improvements in storage density and reliability still need to be resolved.c. ApplicationFRAM is a non-volatile memory that combines the advantages of low power consumption, high speed, long service life, and anti-radiation. It is promising in RF 1C card, fast startup memory, and system chip of cache and aerospace.d. Commercial progressFrom the point of international respect, well-known American company Ramtorn, which developed the first 4K bit commercial ferroelectric memory in 1993; after 1998, Ramtorn focused on product research and development, and handed over all production to semiconductor manufacturers. With the time goes by, Ramtorn represents the highest level of PZT-based commercial ferroelectric memory. Many other countries started late in the field of ferroelectric research and mainly based on scientific research. For example, the main work of some countries is still the preparation of ferroelectric thin films. MRAMa. MRAM BasicsMRAM is a non-volatile memory. For the performance, the write speed of MRAM is extremely fast, almost 1000 times that of flash memory, and 20 times that of FRAM. And it has unlimited read and write times, also it can switch on and off instantly and extend the battery life of portable computers. In terms of manufacturing and technology, the circuit of MRAM is simpler than ordinary memories, and only one readout circuit is needed for chip access. In addition, MRAM is easier to integrate (only 5 photomask layers are needed in the entire process), and there is no need to redesign at the transistor level of flash memory. All other core technologies used in the design can remain the same and work consistently.From 1986 to 1988, Albert Fert and Peter Grünberg discovered that nano-multilayer films composed of alternating ferromagnetic and non-magnetic metal layers made of molecular beam epitaxy have a much larger size than AMR, which is named as giant magnetoresistance (GMR). GMR is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers, but it was difficult to put into practice. Soon, further research by Parkin team found that the (ferromagnetic / non-magnetic metal / ferromagnetic) three-layer film made by sputtering technology has a much larger giant magnetic resistance at room temperature than a single-layer ferromagnetic metal. For the spin valve, opened the way for practical use of GMR.The discovery and research of GMR had led to the realization of high-sensitivity read heads in high-density disks, and promoted the development of the entire modern hard disk industry. The earliest application of spin valve sensors in hard disk read heads was in the IBM Deskstar 16GP Titan, which was released in 1997 and has a storage capacity of 16.8GB. In 2007, Hitachi introduced the Deskstar 7K1000, the first 1TB hard drive.b. MRAM LimitationMRAM is much higher than flash memory in the production costs.c. MRAM ApplicationWith the advantages of low power consumption, high-speed reading, high integration, radiation resistance and unlimited rewrites, MRAM is used in storage, industrial automation, gaming, energy management, communications, consumption electronic,transportation and avionics fields. In addition, the IOT and big data analytics are gradually emerging,ubiquitous sensor terminals need to collect massive amounts of data, in order to save storage power consumption, MRAM and STT-MRAM have become the better choices for their relatively good performance.d. MRAM Commercial ProgressIn 2006, Freescale launched the world’s first commercial MRAM product with a capacity of 4Mb. Judging from the current product specifications and development status, the use of MRAM is still limited to some specific markets. From the perspective of cost and capacity, it cannot compete with NAND flash memory with a maximum capacity of 8Gb and DRAM with 512Mb capacity. However, with Samsung, Intel, TSMC and Global Foundry and other integrated circuit leaders strengthening investment in R & D and related production lines, STT-MRAM is gradually begin mass production, partially replacing SRAM and DRAM products and becoming one of the mainstream memories. OUMa. OUM BasicPhase change memory is a kind of memory that realizes information storage through material phase change. It is the non-volatile and large-capacity storage technology advocated by Intel, the world's number one semiconductor chip manufacturer. In terms of performance, it has a long read and write operation life and is easier to integrate than flash memory. OUM memory cells are extremely dense, and read operations is more safer than other memories. Low energy consumption, requiring very low power to operate. In addition, OUM unit can write about 1 billion times, which makes it an ideal alternative to large-capacity memory in portable devices. From the aspect of manufacturing process, compared with the integration of existing logic circuits, its storage unit is only 1/3 of MRAM and FRAM, and its production cost is lower than other new memories.Crystalline Phase and Amorphous Phase ChangeAlthough phase change memory is often categorized as “new memory”, the concept of “phase change” has introduced over 50 years. In 1962, the phase transition of As-Te glass was discovered. In 1968, Stanford Robert Ovshinsky described in an article that certain semiconductor materials can rapidly switch between two different states of resistance and conductivity under the action of an electric field (on the order of 10μs), he utilized chain structures, cross links, polymeric concepts, and divalent structural bonding with a huge number of unbonded lone pairs to achieve what is now referred to as the “Ovshinsky Effect”, an effect that turns special types of glassy, thin films into semiconductors upon application of low voltage. This discovery directly led to a large number of subsequent studies on the phase transition of thin films based on tellurium-arsenide-germanium-silicon alloy materials or sulfur-based glasses. In 1970, Nevill and Gordon Moore demonstrated the world’s first 256-bit phase change memory, and Moore was later known for putting forward the famous “Moore's Law” about the number of transistors in a dense integrated circuit doubles about every two years and served as the co-founder of Fairchild Semiconductor and CEO of Intel. After that, research on semiconductor memories based on phase change materials has gradually slowed down due to issues such as materials and power consumption, but phase change materials have been used very successfully in rewritable optical discs such as CD-RW / DVD-RW.   b. OUM LimitationThe read and write speed and frequency of OUM are not as good as FRAM and MRAM, and how to maintain its driving temperature stably is also a big technical problem.c. OUM ApplicationPhase change memory is suitable for wired and wireless communication equipment, consumer electronics, PC and other embedded applications due to its fast read and write speed, strong upgrade capabilities, and low power consumption. For example, it is used in the aerospace embedded system and used in smart meters to further integrate its storage architecture.d. OUM Commercial ProgressPhase change memory, as one of the most promising new memories, can be embedded at all levels of the memory architecture. Because of the similarity between phase change memory and dynamic memory, especially its lower power consumption and scalability, it has been considered as the best substitute for dynamic memory. But phase change memory also has disadvantages. The first is its limited times of erases and writes (usually only 107 to 108). If the number of erasing and writing exceeds this limit, the life of the memory cell will end, and the device can no longer be used. The second disadvantage is the limited write speed. The write speed of phase change memory is 6-10 times slower than dynamic memory. Write Operation of Phase Change Memory UnitNevertheless, phase change memory still has good applications in some fields. Several related studies have proposed various methods to overcome these shortcomings. For example, an architecture adjustment is used to compensate for the loss caused by the performance of writing, which can greatly reduce power consumption, thereby accelerating the commercialization of phase change memory as the main memory of a computer.In addition, the multi-level cell technology has been successfully implemented on phase change memory. In the design of a multi-level cell phase change memory, 2N resistive states are used to represent N digits, respectively, in other words, in a 2-bit multi-level cell phase change memory, 00, 01, 10, and 11 can be represented by four different resistance values, respectively. In the specific design, the resistance of the phase change material can be changed by adjusting the amplitude and time of the writing current / voltage. PFRAMa. PFRAM BasicsPFRAM is a plastic, polymer-based, and non-volatile memory. In terms of performance, PFRAM has advantages such as good stability and low power consumption. From manufacturing process, high density can be obtained through three-dimensional stacking technology, which is easy to make and has extremely low manufacturing costs, only about 10% of NOR-type flash memory.b. PFRAM LimitationPFRAM has a limited read and write operation life and its reads are destructive.c. PFRAM ApplicationThe final product of (PFRAM will be an all-organic storage system, which will be suitable for personal computers, handheld computers, digital cameras, mobile phones, handheld radios and communication devices, GPS systems, audio, video, game background program and other important fields.d. PFRAM Commercial ProgressPFRAM develops slowly in commercial use, and Intel is in a leading position. In 2014, Intel recruited JonKrueger (architecture and software engineer) to work for its polymer memory group and greatly promote the development of multi-layer plastic memory, finally their work is close to the software development stage, which indicates that this memory technology will accelerate to the market.Memory technology will continue to improve to meet different applications. On the one hand, The new type memories will create a new market and enter various application markets, on the other hand, it involves new materials and research concepts, it will be difficult to become the mainstream of the market in a long time. However, in the aerospace, industrial automation,  embedded cache of system chip and other sub-application areas, the new non-volatile memory will gradually transfer its technological breakthroughs to market penetration and achieve rapid development.With the advent of the 5G era, the development of application markets such as the IOT, artificial intelligence, and smart cities, and the urgent need for diverse memory requirements, coupled with traditional memory market change, new type memories will play an increasingly important role in the market. Ⅳ Questions Related to Non-volatile Memory and Going Further 1. Which memory is called non-volatile memory?Examples of non-volatile memory include read-only memory (see ROM), flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy discs and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards. 2. Which is non-volatile memory RAM or ROM?RAM, which stands for random access memory, and ROM, which stands for read-only memory, are both present in your computer. RAM is volatile memory that temporarily stores the files you are working on. ROM is non-volatile memory that permanently stores instructions for your computer. 3. What is another name for non-volatile memory?Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. 4. Which is the most non-volatile memory?NAND flash, the most common type used in data storage, includes several variants, such as single-level cells or one bit per multi-level cell or two bits per cell; three-level cells or three bits per cell and quad-level cells or four bits per cell, respectively. 5. What is non-volatile memory explain with example?NV or non-volatile memory is a term used to describe memory or storage that is saved regardless if the computer has power. ... An example of non-volatile memory and storage is a computer hard drive, flash memory, and ROM.