MBed and a PCF8574

The PCF8574 is an 8 bits I/O port expander that uses the I2C protocol. Using this IC, you can use only the SDA and SCL pins of your Arduino board to control up to 8 digital I/O ports.

A0,A1,A2 are address pins
P0,P1,P2,P3,P4,P5,P6,P7 are digital I/O ports
SDA,SCL are the I2C pins

pcf8574n_pinout

If we set pins A0 to A2 to GND, our device address in binary will be 0x20, thats exactly what I did in my example. To enable read and write there are different values required you can see these in the image below

Schematic

Note that the PCF8574 is a current sink device so you do not require the current limiting resistors

I have only shown 2 LEDs, also note that I have shown A0, A1 and A2 tied to GND, this is what my test module had selected.

Code

This example flashes LEDs on and off

You will need to add the https://developer.mbed.org/users/simon/code/PCF8574/ library to your compiler, if you have signed up there is an Import to compiler button on the page,

[codesyntax lang=”cpp”]

#include "mbed.h"
#include "PCF8574.h"

PCF8574 io(p28,p27,0x40);

int main() {
 while(1) {
 io.write(0xAA);
 wait(0.5);
 io.write(0x55);
 wait(0.5);
 }
}

[/codesyntax]

Link
5PCS PCF8574P DIP-16 Remote 8-bit I/O Expander IC