Arduino and a PCF8574 expander

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.

This 8-bit input/output (I/O) expander for the two-line bidirectional bus (I²C) is designed for 2.5-V to 6-V V_CC operation.

The PCF8574 device provides general-purpose remote I/O expansion for most microcontroller families by way of the I²C interface [serial clock (SCL), serial data (SDA)].

The device features an 8-bit quasi-bidirectional I/O port (P0–P7), including latched outputs with high-current drive capability for directly driving LEDs. Each quasi-bidirectional I/O can be used as an input or output without the use of a data-direction control signal. At power on, the I/Os are high. In this mode, only a current source to V_CC is active.

Features

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

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

pcf8574 pinout

Schematic

Note that the PCF8574 is a current sink device so you do not require the current limiting resistors but my test board had these , doesn't do any harm just means the LEDs are a bit dimmer

Arduino and PCF8574

Code

This example flashes the connected LEDs

[codesyntax lang=”cpp”]

#include <Wire.h>

// address of PCF8574 IC
#define PCF8574_ADDR (0x20)

void setup() 
{
  Wire.begin();
}

void loop() 
{

  
  //send the data
  Wire.beginTransmission(PCF8574_ADDR);
  Wire.write(0xAA);
  Wire.endTransmission();
  delay(1000);
  Wire.beginTransmission(PCF8574_ADDR);
  Wire.write(0x55);
  Wire.endTransmission();
  delay(1000);
}

[/codesyntax]

Link

Datasheet – http://www.ti.com/lit/gpn/pcf8574