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PCA9665 in Practice: Solving I2C Hang-ups and Optimizing Buffer Performance

  • Contents

Quick-Reference Card: PCA9665 at a Glance

Attribute Detail
Component Type I2C Controller / Parallel-to-I2C Bus Converter
Manufacturer NXP USA Inc.
Key Spec 1 Mbit/s Fast-mode Plus (Fm+) support
Supply Voltage 2.3 V to 3.6 V
Package Options HVQFN20
Lifecycle Status Active
Best For Adding I2C ports to parallel-bus MCUs or processors

PCA9665 product photo in HVQFN20 package


1. What Is the PCA9665? (Definition + Architecture)

The PCA9665 is a parallel-bus to I2C-bus protocol converter from NXP USA Inc. that enables microcontrollers without native I2C peripherals to communicate via high-speed serial interfaces. While many modern MCUs include integrated I2C blocks, the PCA9665 remains a vital bridge for legacy systems, FPGA-based designs, or applications requiring additional, independent I2C ports without bit-banging.

1.1 Core Architecture & Design Philosophy

The PCA9665 functions as an intermediary between an 8-bit parallel data bus and the serial I2C-bus. Internally, it features a sophisticated state machine that handles the complex timing of I2C protocols (Start, Stop, Ack/Nack, and Arbitration) which offloads significant processing overhead from the host CPU. Unlike its predecessor (the PCF8584), the PCA9665 is optimized for "Fast-mode Plus" (Fm+), allowing it to drive heavily loaded buses at higher speeds.

1.2 Where It Fits in the Signal Chain

This component sits directly between the host processor's address/data bus and the external I2C peripheral network. It acts as a "Protocol Translator." Upstream, it interfaces with standard 8-bit parallel signals (CS, RD, WR, and Data); downstream, it drives the SDA and SCL lines to communicate with sensors, LCDs, or EEPROMs.


2. Electrical Characteristics: The Numbers That Matter

2.1 Power Supply & Consumption Profile

The PCA9665 operates on a 2.3 V to 3.6 V supply. This makes it ideal for modern 2.5V and 3.3V logic levels. However, a standout feature is its 5 V tolerant I/Os. This allows the device to interface with 5V legacy parallel buses while running on a lower voltage rail, eliminating the need for external level shifters in many mixed-voltage designs.

2.2 Performance Specs (Speed & Drive)

  • 1 Mbit/s Fast-mode Plus: This is a 2.5x speed increase over standard Fast-mode (400 kHz).
  • 25 mA Drive Capability: The SDA and SCL pins can sink up to 25 mA. Why it matters: This allows the PCA9665 to drive long PCB traces or buses with high capacitance (up to 4000 pF) without violating I2C rise/fall time requirements.

2.3 Absolute Maximum Ratings — What Will Kill It

  • VCC range: -0.5V to +4.6V.
  • Input Voltage (any pin): -0.5V to +6.0V. (Note the 5V tolerance, but do not exceed 6V).
  • Total Power Dissipation: Ensure the HVQFN20 package thermal pad is properly soldered to manage heat if driving high-capacitance buses at 1 Mbit/s.

3. Pinout & Package Guide

PCA9665 HVQFN20 pinout diagram

3.1 Pin-by-Pin Functional Groups

Pin Group Pins Function
Power VDD, VSS Supply and Ground
Parallel Data D0–D7 8-bit bidirectional data bus
Control CS, RD, WR Chip Select, Read, and Write strobes
Addressing A0, A1, A2 Internal register selection
I2C Interface SDA, SCL Serial data and clock lines
Interrupt INT Active LOW interrupt output to host

3.2 Package Variants & Soldering Notes

The PCA9665 is primarily available in the HVQFN20 (Heatsink Very thin Quad Flat-pack No-leads) package. * Soldering Note: The center thermal pad must be soldered to the PCB ground plane. In high-density boards, smearing can occur during paste printing. It is critical to use a laser-cut stencil and a reflow profile that ensures the peak temperature is maintained long enough for the center pad to wet properly.


4. Known Issues, Errata & Real-World Pain Points

4.1 I2C Bus Hang-up

Problem: During specific master-transmitter sequences, the I2C bus may fail to release the SCL or SDA lines, effectively "hanging" the bus. Root Cause: This often occurs during unexpected arbitration loss or improper state transitions. Recommended Fix: Implement a "dummy write" to the internal control register to clear the state machine. Additionally, leverage the software reset feature via the parallel bus to re-initialize the device without a hard power cycle.

4.2 Buffer Underrun Issues

Problem: The device may send a premature STOP condition during high-speed transfers. Root Cause: If the host CPU is too slow to refill the data register, the PCA9665 assumes the transmission is complete. Recommended Fix: Utilize the 68-byte hardware buffer. Instead of byte-by-byte transfers, load the buffer entirely before triggering the I2C start. This decouples the host's parallel bus latency from the I2C timing.


5. Application Circuits & Integration Examples

5.1 Typical Application: Adding I2C to a Legacy 8051 MCU

In this scenario, the PCA9665 is mapped into the 8051's external memory space. The A0–A2 lines determine which internal register (Control, Data, or Status) the MCU is accessing.

5.2 Interface Example: Initialization Sequence

To begin communication, the host must configure the clock frequency and the mode.

// Example Initialization Sequence
write_reg(INDPTR, 0x00);      // Point to Indirect Register 0
write_reg(CONTROL, 0x80);    // Reset the PCA9665
write_reg(CLOCK_RATE, 0x05); // Set for 400kHz or 1MHz operation
write_reg(OWN_ADDR, 0xAA);   // Set slave address (if used in slave mode)

6. Alternatives, Replacements & Cross-Reference

6.1 Pin-Compatible & Functional Alternatives

Part Number Manufacturer Key Difference Compatible?
PCF8584 NXP Older version, slower (100kHz), different pinout. ?
SC16IS750 NXP I2C/SPI to UART/GPIO; different focus. ??
TCA9517 TI Level shifter/repeater (not a parallel bridge). ?

6.2 Upgrade Path

If your design requires even higher speeds or more channels, consider moving to a modern MCU with multiple integrated I2C/Fm+ peripherals (such as the NXP LPC or Kinetis series), which eliminates the need for a bridge chip entirely.


7. Procurement & Supply Chain Intelligence

  • Lifecycle Status: Active. NXP continues to support this part for industrial and legacy automotive applications.
  • Typical MOQ: Standard reels are usually 2,500 pieces, but "Cut Tape" is widely available for prototyping.
  • BOM Risk Factors: The PCA9665 is a relatively niche, single-source component. While supply is generally stable, designers should maintain a small safety stock for long-lifecycle industrial products.
  • Authorized Distributors: Available via Mouser, Digi-Key, and Arrow. Avoid unverified third-party brokers due to the risk of counterfeit HVQFN parts.

8. Frequently Asked Questions

Q: What is the PCA9665 used for? A: It is primarily used to add I2C-bus ports to microcontrollers that lack them or to add additional high-speed (1 Mbit/s) ports to a system that has already exhausted its native I2C resources.

Q: What are the best alternatives to the PCA9665? A: There are few direct "parallel-to-I2C" bridges left. Most engineers either use a small "bridge MCU" (like an ATTiny) or move to a more modern host processor from TI or STMicroelectronics that includes native Fm+ I2C support.

Q: Can the PCA9665 work with 3.3V logic? A: Yes. It is designed for a 2.3V to 3.6V supply range and is fully compatible with 3.3V logic levels on both the parallel and serial sides.


9. Resources & Tools

  • Official Datasheet: [NXP PCA9665 Product Page]
  • Evaluation Board: OM13210 (NXP Fm+ Development Kit)
  • Reference Designs: See NXP AN10658 for "Sending/Receiving I2C-bus data with the PCA9665."

PCA9665BS,118 Documents & Media

Download datasheets and manufacturer documentation for NXP USA Inc. PCA9665BS,118.
PCN Obsolescence/ EOL
Datasheets

PCA9665BS,118 PCB Symbol, Footprint & 3D Model

NXP USA Inc. PCA9665BS,118

NXP USA Inc.

Parallel Bus to I2C Bus Controller 2.5V/3.3V 20-Pin HVQFN EP T/R

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