I2C 16bit IO extender

The PCA9555 is a 16 bit IO chip which interfaces with I2C. Each of the 16 bits can be individually configured as a digital input or digital output.

PCA9555 demo board. GPIO pins are on the top.

I will be using this to control a bank of relays from my Raspberry Pi. If you are not familiar with I2C on the RPI you might want to look at my posts about the PCA8591.

This time I bought the chip mounted to a board from ebay. I had to solder the header pins to the board so I could connect all the inputs & outputs.

jnewbigin@raspberrypi:~$ i2cdetect 1

WARNING! This program can confuse your I2C bus, cause data loss and worse!

I will probe file /dev/i2c-1.

I will probe address range 0x03-0x77.

Continue? [Y/n]

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f

00:          -- -- -- -- -- -- -- -- -- -- -- -- --

10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

70: -- -- -- -- -- -- -- --          

As expected, the chip shows up at address 0x20. The PCA9555 is programmed by a set of registers. The registers are split up into ports or banks which operate on 8 bits at a time. The registers are:

• 0x00 Select Input values for bank 0
• 0x01 Select Input values for bank 1
• 0x02 Select Output values for bank 0
• 0x03 Select Output values for bank 1
• 0x04 Select Polarity Inversion for bank 0
• 0x05 Select Polarity Inversion for bank 1
• 0x06 Select IO Configuration for bank 0
  (set/1 = input (default), clear/0 = output)
• 0x07 Select IO Configuration for bank 1
  (set/1 = input (default), clear/0 = output)

To read the first 8 inputs

i2cset -y 1 0x20 0x00

i2cget -y 1 0x20

To read the next 8 inputs

i2cset -y 1 0x20 0x01

i2cget -y 1 0x20

To set the first 8 bits to be outputs

i2cset -y 1 0x20 0x06 0x00

And to turn all the outputs on

i2cset -y 1 0x20 0x02 0xff

My next job is to hook up the relay boards but that requires setting up a more permanent power supply which can run the Raspberry Pi, drive the relays and power the relay devices. Hopefully photos to come soon.