PCF8574A Remote 8-BIT I/O Expander For I2C Bus

Datasheet: http://mirror-sg.icy.com.au/datasheets/ti/pcf8574a.pdf

Store: Buy PCF8574AN

What can I use it for?

This chip lets you use two communication pins on a micro-controller to give you an extra 8 pins of general purpose input and output. Furthermore, each pin can sink over 100mA per pin, so it also acts as a buffer or driver IC for output. This is particularly useful for driving LEDs, or other lighting, small motors, etc. directly from this chip.

How does it work?

This IC communicates over the TWI or I2C bus, which are often found as the two pins labelled SDA (data) and SCL (clock) on a micro-controller. It acts as a "slave device", where your micro-controller would be a master that sends or asks it for data. This particular method of communication allows you to share these two pins with many TWI/I2C devices. This means if you have 5 of these ICs, you can use two micro-controller pins to control 40 input/output pins. The micro-controller knows which device it is talking to, by identifying each I2C device by an address. This address can be set by connecting three of the pins on this IC to Vcc (+) or Gnd (-). This gives 8 combinations, allowing 8 different addresses (thus 8 of these ICs can be connected to the one I2C bus).

What else do I need to know?

The I2C/TWI bus requires two pull-up resistors connecting the SDA and SCL lines to Vcc. For a 5V device, something around the 4.7K value works well. Some micro-controllers have built-in pullups that can be enabled. If these don't work (they might not pull up enough, or pull up too much) then you can attach external resistors to the two lines.

There is also an interrupt line on this IC that outputs a value back to the micro-controller when the value of one of its pins changes. This lets the master know when something's changed, without the need for this IC to become a master (it can stay slave).

Can I use it with Arduino or AVR Microcontrollers?

Yes. Arduino comes with the "Wire" library specifically for this purpose. The datasheet for this chip tells us that if we connect all the address pins to Gnd, then its address will be 56 (decimal) or 0x38 (hexidecimal). SDA on arduino is Analog Port, Pin 4 and SCL is Analog Port, Pin 5. These connect to SDA and SCL on the ICs (with pull-up resistors, if required).

The following Arduino code will turn on (output high) on each pin of the IC in turn, and then read the input value of all 8 pins: 

   1 #include <Wire.h>
   2 
   3 void setup( ) {
   4     Serial.begin( 9600 );
   5     Wire.begin( ); // initialise TWI as a master
   6 }
   7 
   8 void loop( ) {
   9     char portData;
  10     
  11     // set each pin in turn
  12     for ( int i = 0; i != 8; ++i ) {
  13         // send a byte where only the i'th bit is set
  14         // each bit represents a pin on the 8-pin port
  15         portData = 0;
  16         bitSet( portData, i ); // set the i'th bit on the port
  17         Wire.beginTransmission( 0x38 ); // the address assigned to the extender IC
  18         Wire.send( portData );
  19         Wire.endTransmission( );
  20         
  21         delay( 500 );
  22     }
  23     
  24     Wire.requestFrom( 0x38, 1 ); // request 1 byte from device at address 0x38
  25     if ( Wire.available( ) > 0 ) {
  26         portData = Wire.receive( );
  27     }
  28     
  29     // print:
  30     // P0=0 or 1 (depending on whether this pin is low or high)
  31     // P1=0 or 1, etc.
  32     for ( int i = 0; i != 8; ++i ) {
  33         Serial.print( "P" );
  34         Serial.print( i );
  35         Serial.print( "=" );
  36         Serial.println( bitRead( portData, i ) );
  37     }
  38     
  39     Serial.write()
  40 }

For other AVR micro-controllers, here are some links to I2C/TWI libraries and code:

Troubleshooting

Guides/PCF8574A (last edited 2010-12-03 21:58:09 by TheoJulienne)