pcf8574 v0.1.0

PCF8574 8-bit I2C GPIO expander.

Application scenarios

• A few buttons or relays on a small MCU.
• A status LED bank.
• A slow control panel.

Contract

uint8_t pcf8574_init(pcf8574_t *device, pcf8574_port_t port, uint8_t i2c_address);
uint8_t pcf8574_write_port(pcf8574_t *device, uint8_t value);
uint8_t pcf8574_read_port(pcf8574_t *device, uint8_t *value);
uint8_t pcf8574_write_pin(pcf8574_t *device, uint8_t pin, uint8_t value);
uint8_t pcf8574_read_pin(pcf8574_t *device, uint8_t pin, uint8_t *value);

• Pins are 0..7.
• A written 1 releases the pin; an external driver can then pull it low.
• write_pin updates a shadow latch, so other output pins keep their state.
• Returns 1 on success, 0 on a bad pin or a bus error.

Parameters

• I2C address 0x20..0x27 from A2/A1/A0.
• There is no direction register; use open-drain drive or a series resistor.
• The reset state is all ones (inputs released).

Memory and timing

• 4 bytes; one I2C write or read.
• A pin update is one byte on the bus.

Integration

pcf8574_t expander;
pcf8574_init(&expander, board_i2c_port(), 0x20);
pcf8574_write_pin(&expander, 0, 0); /* drive low */
pcf8574_write_pin(&expander, 0, 1); /* release */

Adapters

• The adapter writes or reads one byte, with no register pointer.
• Reference: LibDriver pcf8574 (MIT) and the Texas Instruments PCF8574 datasheet.
• This implementation is original.

When not to use

• A pin that needs a direction register or interrupt masking; use MCP23017.
• A fast signal; the I2C update rate is the limit.

Verification

make test covers init, port writes, pin updates, reads, and invalid pins.

void *context;
uint8_t (*write)(void *context, uint8_t address, const uint8_t *data, uint16_t length);
uint8_t (*read)(void *context, uint8_t address, uint8_t *data, uint16_t length);

pcf8574_port_t port;
uint8_t i2c_address;
uint8_t output;