MicroPython PCF8574

A MicroPython library for PCF8574 8-Bit I2C I/O Expander with Interrupt.

The PCF8574 consists of a 8-bit quasi-bidirectional port and an I2C-bus interface.

The device includes latched outputs with high current drive capability for directly driving LEDs.

The interrupt has an open-drain output, which means you need a pull-up on your microcontroller to detect when the PCF8574 drives it LOW.

When setting a pin HIGH, it acts as both output AND input. The pin internally uses a weak current-source pull-up to latch output HIGH. When driven LOW, for example, with a push button, the pin will read as LOW.

An interrupt fires on any rising or falling edge of the pins in input mode (HIGH). Interrupt is cleared when the pins are changed or the port is read.

At power on, all pins are driven HIGH and can be immediately used as inputs.

Operating voltage: 2.5V - 5.5V

Example

Copy the file to your device, using ampy, rshell, webrepl or compiling and deploying. eg.

$ ampy put pcf8574.py

Basic Usage

import pcf8574
from machine import I2C, Pin

# TinyPICO (ESP32)
i2c = I2C(scl=Pin(22), sda=Pin(21))
pcf = pcf8574.PCF8574(i2c, 0x20)

# read pin 2
pcf.pin(2)

# set pin 3 HIGH
pcf.pin(3, 1)

# set pin 4 LOW
pcf.pin(4, 0)

# toggle pin 5
pcf.toggle(5)

# set all pins at once with 8-bit int
pcf.port = 0xff

# read all pins at once as 8-bit int
pcf.port

For more detailed examples, see examples.

Pins

Pin	Type	Description
A0	I	Address select 1, connect to VCC or GND
A1	I	Address select 2, connect to VCC or GND
A2	I	Address select 3, connect to VCC or GND
INT	O	Interrupt output, open drain, active LOW
P00	I/O	Port A, Pin 0
P01	I/O	Port A, Pin 1
P02	I/O	Port A, Pin 2
P03	I/O	Port A, Pin 3
P04	I/O	Port A, Pin 4
P05	I/O	Port A, Pin 5
P06	I/O	Port A, Pin 6
P07	I/O	Port A, Pin 7
SDA	I/O	I2C Serial Data, needs pull-up
SCL	I	I2C Serial Clock, needs pull-up
GND	PWR	Ground
VCC	PWR	Supply voltage 3.3-5V

Methods

Construct with a reference to I2C and set the device address. Valid address range 0x20-0x27. If are you not sure what it is, run an i2c.scan(). See below for address selection.

__init__(i2c, address=0x20)

Method for getting or setting a single pin. If no value is provided, the port will be read and value of specified pin returned. If a value is provided, the port will be updated and device written to. The port is written to after each call. If you intend to toggle many pins at once, use the port property instead. See below. Valid pin range 0-7.

pin(pin, value=None)

Method for flipping the value of a single pin. Valid pin range 0-7.

toggle(pin)

Private method for loading _port from the device.

_read()

Private method for sending _port to the device.

_write()

Properties

Getter reads the port from the device and returns a 8-bit integer.

port

Setter writes an 8-bit integer representing the port to the device. If you are setting multiple pins at once, use this instead of the pin() method as this writes the entire 8-bit port to the device once, rather than 8 separate writes.

port = 0xffff

Ports

• P00-P07 - Port A

This chip only has one port. If you need more pins, the PCF8575 has two ports (another 8 pins).

Interrupts

• INT - Active LOW

I2C Interface

If you are using a module, most contain 10k pull-ups on the SCL and SDA lines.

If you are using the PCF8574 chip directly, you'll need to add your own.

I2C Address

The chip supports I2C addresses 0x20-0x27 and is customisable using address pins A0, A1, A2

A0	A1	A2	I2C Address
GND	GND	GND	0x20 (default)
3V3	GND	GND	0x21
GND	3V3	GND	0x22
3V3	3V3	GND	0x23
GND	GND	3V3	0x24
3V3	GND	3V3	0x25
GND	3V3	3V3	0x26
3V3	3V3	3V3	0x27

Parts

• TinyPICO $20.00 USD
• PCF8574 10x DIP $3.00 AUD
• PCF8574 blue board $0.83 AUD
• PCF8574 red board $0.95 AUD
• PCF8574 purple board $0.99 AUD
• PCF8574 as LCD backpack $0.98 AUD

Connections

TinyPICO	PCF8574 Module
21 SDA	SDA
22 SCL	SCL
3V3	VCC
G	GND
4	INT (optional)

Links

• TinyPICO Getting Started
• micropython.org
• PCF8574 datasheet
• Adafruit Ampy

License

Licensed under the MIT License.

Copyright (c) 2019 Mike Causer