pca9674a — I2C Expander Driver

The pca9674a module provides a driver for the NXP PCA9674A 8-bit quasi-bidirectional I/O expander. The chip exposes an 8-bit I/O port over I2C and an active-low INT line that pulses whenever any input pin changes state.

Each pin is quasi-bidirectional: writing 1 releases the pin to a weak internal pull-up so it acts as an input that an external driver can pull low; writing 0 actively drives the pin low. There is no separate direction register – PCA9674A.read() returns the current external level of every pin regardless of what was last written.

The 7-bit I2C address is 0x38 – 0x3F depending on the A0/A1/A2 strap pins; 0x3F (63) is the default (all-high straps).

class PCA9674A – 8-bit I/O expander

class pca9674a.PCA9674A(bus: machine.I2C, irq_pin: str, address: int = 63, callback: Callable | None = None)

Create an interface to a PCA9674A I/O expander.

  • bus is the machine.I2C bus the expander is connected to.

  • irq_pin is the pin label on the host MCU connected to the expander’s INT output. The driver configures it as an input with a pull-up and watches for its falling edge.

  • address is the 7-bit I2C address of the expander (0x38 – 0x3F). Defaults to 0x3F.

  • callback is invoked on the falling edge of irq_pin when any pin state changes. It receives the PCA9674A instance as its only argument; the new pin state can be read with read(). Pass None (the default) for polled use.

write(value: int) → None

Writes the 8-bit value to the I/O expander pins.

read() → int

Returns the 8-bit value of the I/O expander pins.

reset() → None

Resets and re-initializes the I/O expander, and re-attaches the IRQ callback if one was provided.