PCF8574/PCF8575 I2C Driver

A Mongoose library for PCF857X, a popular and cheap set of GPIO extenders using I2C. The PCF8574 is an 8-port device, and the PCF8575 is a 16-port device, but they are otherwise identical.

Implementation details

The PCF857X is a pseudo bi-directional GPIO, which means that by default ports are input and output. Writes to the ports can change the state from high to low. If the port is high, it is pulled up by a weak current. Each port can be pulled low by an external device, in which case reads from that port return 0. The implication is that "input" ports are always pulled high.

API Description

To start, mgos_pcf857x_create() is called with the correct I2C bus and address (by default 0x20), and optionally a GPIO pin on the microcontroller that serves as an interrupt pin, to detect PCF857X input state changes.

NOTE: When the driver starts, it polls the current state from the chip without changing any ports. The benefit of this is that the MCU can safely reboot without loss of the GPIO state in PCF857X.

The API follows mgos_gpio.h closely, enabling ports to be set as input (ignoring the pull_type, which is always pullup), or as output. Notably, mgos_pcf857x_gpio_set_int_handler() and mgos_pcf857x_gpio_set_button_handler() work identically to the mgos_gpio_*() variants.

Example application

#include "mgos.h"
#include "mgos_config.h"
#include "mgos_pcf857x.h"

static void button_cb(int pin, void *user_data) {
  struct mgos_pcf857x *d = (struct mgos_pcf857x *)user_data;
  LOG(LL_INFO, ("GPIO[%d] callback, value=%d", pin, mgos_pcf857x_gpio_read(d, pin)));
  mgos_pcf857x_gpio_toggle(d, pin+4);
}

enum mgos_app_init_result mgos_app_init(void) {
  struct mgos_pcf857x *d;
  int i;

  if (!(d = mgos_pcf8574_create(mgos_i2c_get_global(), mgos_sys_config_get_pcf857x_i2caddr(),
                                mgos_sys_config_get_pcf857x_int_gpio()))) {
    LOG(LL_ERROR, ("Could not create PCF857X"));
    return MGOS_APP_INIT_ERROR;
  }

  for(i=0; i<4; i++) mgos_pcf857x_gpio_setup_input(d, i, MGOS_GPIO_PULL_UP);
  for(i=4; i<8; i++) mgos_pcf857x_gpio_set_mode(d, i, MGOS_GPIO_MODE_OUTPUT);

  mgos_pcf857x_gpio_set_button_handler(d, 0, MGOS_GPIO_PULL_UP, MGOS_GPIO_INT_EDGE_NEG, 10, button_cb, d);
  mgos_pcf857x_gpio_set_button_handler(d, 1, MGOS_GPIO_PULL_UP, MGOS_GPIO_INT_EDGE_POS, 10, button_cb, d);
  mgos_pcf857x_gpio_set_button_handler(d, 2, MGOS_GPIO_PULL_UP, MGOS_GPIO_INT_EDGE_ANY, 10, button_cb, d);
  mgos_pcf857x_gpio_set_button_handler(d, 3, MGOS_GPIO_PULL_UP, MGOS_GPIO_INT_EDGE_ANY, 10, button_cb, d);

  mgos_pcf857x_gpio_blink(d, 4, 1000, 1000);

  return MGOS_APP_INIT_SUCCESS;
}

Disclaimer

This project is not an official Google project. It is not supported by Google and Google specifically disclaims all warranties as to its quality, merchantability, or fitness for a particular purpose.