Interrupts on the PiFace

I recently acquired both a Raspberry Pi and a PiFace IO board. I had a rough time finding examples of how to read the input ports via interrupts (rather than periodically polling for values), especially for the newer versions of the PiFace python libraries.

After a little research, here’s some simple code that will print out pin names as you press the input buttons. Button 3 will cause the code to exit:

#!/usr/bin/python

import pifacecommon.core
import pifacecommon.interrupts
import os
import time

quit = False

def print_flag(event):
    print 'You pressed button', event.pin_num, '.'

def stop_listening(event):
    global quit
    quit = True

pifacecommon.core.init()

# GPIOB is the input ports, including the four buttons.
port = pifacecommon.core.GPIOB

listener = pifacecommon.interrupts.PortEventListener(port)

# set up listeners for all buttons
listener.register(0, pifacecommon.interrupts.IODIR_ON, print_flag)
listener.register(1, pifacecommon.interrupts.IODIR_ON, print_flag)
listener.register(2, pifacecommon.interrupts.IODIR_ON, print_flag)
listener.register(3, pifacecommon.interrupts.IODIR_ON, stop_listening)

# Start listening for events.  This spawns a new thread.
listener.activate()

# Hang around until someone presses button 3.
while not quit:
    time.sleep(1)

print 'you pressed button 3 (quitting)'
listener.deactivate()