IO Expander Interface (PCAL6408A)

Contents

Introduction
PCAL6408A Features
MicroPython Interface
Complete Example
C/C++ Interface
Hardware Connection
Troubleshooting
Register Map

Introduction

The IO Expander interface provides access to the PCAL6408A 8-bit I2C GPIO expander available on SGW3501-F1-StarterKit boards. This component extends the available GPIO pins and provides interrupt-capable I/O expansion.

I2C Bridge Architecture

The IO expander implementation uses MicroPython I2C Bridge architecture for ESP-IDF v5.4+ compatibility:

Driver: PCAL6408A 8-bit I2C GPIO expander
Bridge: mp_i2c_bridge component provides C interface to MicroPython I2C
Benefits: Unified I2C management, modern ESP-IDF compatibility, future-proof

Implementation Details

Device Address: 0x20 (7-bit I2C address)
I2C Pins: SCL=20, SDA=21 (configurable)
Frequency: 100kHz I2C bus frequency
Integration: Uses ioexp_mp.c with I2C bridge for MicroPython builds

PCAL6408A Features

8-bit I2C-bus GPIO with interrupt and weak pull-up
5V tolerant inputs
Polarity inversion register
Low current consumption
Interrupt output for pin change detection
Compatible with standard GPIO operations

MicroPython Interface

Initialization

import ioexp

# Initialize the IO expander
ioexp.init()

Pin Configuration

# Set pin direction (1 = input, 0 = output)
ioexp.set_direction(pin_number, direction)

# Example: Set pin 0 as output, pin 1 as input
ioexp.set_direction(0, 0)  # Output
ioexp.set_direction(1, 1)  # Input

Digital I/O Operations

# Write to output pin
ioexp.write_pin(pin_number, value)

# Example: Set pin 0 high
ioexp.write_pin(0, 1)

# Read from input pin
value = ioexp.read_pin(pin_number)

# Example: Read pin 1 state
state = ioexp.read_pin(1)
print(f"Pin 1 state: {state}")

Port Operations

# Write entire 8-bit port at once
ioexp.write_port(0b10101010)  # Set pins 1,3,5,7 high

# Read entire port
port_state = ioexp.read_port()
print(f"Port state: 0x{port_state:02X}")

Pull-up Configuration

# Enable/disable weak pull-up resistors
ioexp.set_pullup(pin_number, enable)

# Example: Enable pull-up on pin 2
ioexp.set_pullup(2, 1)

Polarity Inversion

# Configure polarity inversion
ioexp.set_polarity(pin_number, inverted)

# Example: Invert polarity on pin 3
ioexp.set_polarity(3, 1)

Complete Example

import ioexp
import time

# Initialize the IO expander
ioexp.init()

# Configure pins: 0-3 as outputs, 4-7 as inputs with pull-ups
for pin in range(4):
    ioexp.set_direction(pin, 0)  # Output

for pin in range(4, 8):
    ioexp.set_direction(pin, 1)  # Input
    ioexp.set_pullup(pin, 1)     # Enable pull-up

# Blink LEDs on output pins
while True:
    # Set outputs high
    for pin in range(4):
        ioexp.write_pin(pin, 1)

    # Read and display input states
    for pin in range(4, 8):
        state = ioexp.read_pin(pin)
        print(f"Input pin {pin}: {state}")

    time.sleep(0.5)

    # Set outputs low
    for pin in range(4):
        ioexp.write_pin(pin, 0)

    time.sleep(0.5)

C/C++ Interface

For native C/C++ applications, the same functionality is available through the header interface:

#include "ioexp.h"

// Initialize
esp_err_t err = ioexp_init();

// Configure pin direction
ioexp_set_direction(0, IOEXP_OUTPUT);
ioexp_set_direction(4, IOEXP_INPUT);

// Digital operations
ioexp_write_pin(0, 1);  // Set pin 0 high
int state = ioexp_read_pin(4);  // Read pin 4

Hardware Connection

The PCAL6408A is typically connected as follows on SGW3501-F1-StarterKit:

VCC: 3.3V power supply
GND: Ground
SCL: I2C clock line (GPIO 20)
SDA: I2C data line (GPIO 21)
INT: Interrupt output (optional, board-dependent)
ADDR: Address select pin (determines I2C address)

Troubleshooting

Common Issues

I2C Communication Errors
- Verify I2C connections (SCL, SDA, power, ground)
- Check I2C address (0x20 default)
- Ensure proper pull-up resistors on I2C lines
Pin State Issues
- Verify pin direction configuration
- Check for polarity inversion settings
- Ensure proper voltage levels (5V tolerance)
Initialization Failures
- Confirm device presence with I2C scan
- Verify power supply stability
- Check for I2C bus conflicts

Debug Commands

# Test I2C communication
import machine
i2c = machine.I2C(0, scl=20, sda=21)
devices = i2c.scan()
print(f"I2C devices found: {[hex(d) for d in devices]}")

# Expected output should include 0x20 for PCAL6408A

Register Map

For advanced users, the PCAL6408A register map:

Register	Address	Function
Input Port	0x00	Read input levels
Output Port	0x01	Write output levels
Polarity Inversion	0x02	Configure input polarity
Configuration	0x03	Set pin direction
Output Drive Strength	0x40	Configure drive strength
Input Latch	0x42	Input latch control
Pull-up/Pull-down Enable	0x43	Pull resistor enable
Pull-up/Pull-down Select	0x44	Pull resistor direction
Interrupt Mask	0x45	Interrupt enable
Interrupt Status	0x46	Interrupt status
Output Port Configuration	0x4F	Output port configuration