PCF8575.h

A Raspberry Pi Pico library for PCF8575 I/O expander modules.

Description

A simple single-file c++ library written for the Raspberry Pi Pico (or any RP2040 board) to communicate with I/O expander modules based on the PCF8575 microchip.

Note

This is a single header file for ease of use in small hobby projects, but this is not best practice and the actual implementation should be placed in an accompanying .cpp file for use in larger or professional projects to reduce compile overhead.

Requirements

The Raspberry Pi Pico SDK

This library can only be used alongside the Raspberry Pi Pico SDK.

Enable I²C

I²C must be enabled by adding hardware_i2c to the list of linked libraries in your CMakeLists.txt, e.g.:

target_link_libraries(your_project pico_stdlib ... hardware_i2c ...)

Include PCF8575.h

PCF8575.h must be included in the source file where it is to be used.

Use

Initialisation

This library provides the class PCF8575. Each expander must first be initialised by creating an instance of this class with the correct parameters.

Initialising a new expander looks something like this:

PCF8575 expander_name(sda_pin, scl_pin, i2c_bus, expander_address);

For example, initialising an expander called myexpander with the default address 0x20 connected to the I²C bus i2c0 on pins 20 and 21 of a Raspberry Pi Pico would look like this:

PCF8575 myexpander(20, 21, i2c0, 0x20);

Setting/Writing

To set the state of one or many pins:

expander_name.write(state_bool, first_pin_int, pin_count_int);		

state_bool must be either 0 (low) or 1 (high).
first_pin_int is the first (or only) pin you wish to set. Pins are numbered from 0 to 15
pin_count_int is the number of pins, counting upwards, to be set to this state. Use 1 to set only first_pin

For example, to set all 16 pins on myexpander to 1 (high):

myexpander.write(1, 0, 16);

To then set only pin 7 to 0 (low):

myexpander.write(0, 7, 1);

Reading

The read() function returns an array of 16 boolean values:

expander_name.read()

Individual pins can be selected by appending [pin_number] to the read() call.

For example, to initialise an array of 16 booleans and then set them to the current state of the I/O expander every time the read() function is called:

// When initialising variables
bool[16] states = {0};

// Later, e.g. in a loop or on an input call
states = myexpander.read();

Or, to get just one value as needed:

bool pin_seven = 0;

pin_seven = myexpander.read()[7];