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Python interface to the Raspberry Pi PiFace board

Project description

PIFACEIO

This package provides a Python interface to the PiFace Digital peripheral I/O board for the Raspberry Pi. A PiFace Digital board offers 8 digital inputs and 8 digital outputs. This package allows a Python program to read the inputs and write the outputs on the board via the Raspberry Pi SPI bus.

The newer PiFace Digital 2 board is exactly compatible with the original board and so is also supported by this package.

Multiple PiFace Digital boards are supported, on either or both of the RPi SPI bus chip selects. This pifaceio package is focussed on simplicity and performance for polled implementations and is an alternative to the pifacedigitalio and piface (now depreciated) Python packages for the PiFace Digital board. In my simple polled read and write benchmarks, pifaceio performs significantly faster and with much less overhead than pifacedigitalio.

Interrupts are not supported. See pifacedigitalio for interrupt and other functionality.

The pifaceio package is implemented in pure Python code using only the Python standard library, uses no external 3rd party packages, and is compatible with Python versions 2 and 3.

INSTALLATION

Install necessary packages on your Raspberry Pi for build etc:

sudo apt-get install git python-setuptools

Get this package:

git clone http://github.com/bulletmark/pifaceio

Install (can alternately do this as ordinary user in a virtualenv of course):

sudo python ./setup.py install

To set up permissions/groups/udev etc for spidev device on RPi, run the following included script and then reboot.

sudo ./install-spidev.sh

Note that the pifaceio pypi package is also available from PyPi so alternatively you can install it using pip (with or without a virtualenv).

USAGE

Board addresses, input pins, and output pins are always numbered from 0.

In general, you start with a once-off allocation of a PiFace board instance at startup with:

pf = pifaceio.PiFace()

Default is first PiFace board (0). Optionally takes an argument 0 to 7 for up to 8 PiFace board addresses. Create multiple PiFace() instances if you want to use multiple boards in parallel.

There are also other (rarely needed) options to disable the input pull up resistors, and to invert the input and output bit polarities. See pifaceio.py for details.

At each poll time, e.g. every part second, read all the inputs (i.e. the single input byte) with:

pf.read() # returns the input byte you can use directly if you prefer

Then read and write individual pins according to your logic with:

in_val = pf.read_pin(pin_in)
..
pf.write_pin(pin_out, out_val)
..

Finally, write all the outputs at the end of processing (i.e. write the single output byte) with:

pf.write() # optionally, takes an output byte to write directly

Note that read_pin() is just a convenience method wrapping a bit test around the previously read input byte from read() and write_pin() is just a convenience method wrapping a bit set/clear around the output byte pending it being written by write(). You don't have to use read_pin() or write_pin() if you just want to read, test/manipulate, and write the 8 bit input and/or output byte directly. In that case you would just use read(), and write() only in your application.

EXAMPLES

Simple example to just reflect all PiFace 8 inputs to the 8 outputs every 10 msec, on the default first PiFace board:

import pifaceio, time
pf = pifaceio.PiFace()

while True:
    pf.write(pf.read())
    time.sleep(.01)

Same example, but do it across 4 PiFace boards:

import pifaceio, time
pifaces = [pifaceio.PiFace(n) for n in range(4)]

while True:
    for pf in pifaces:
        pf.write(pf.read())
    time.sleep(.01)

Simple example to test if both input pin 0 and 1 are on at same time, and then set output pin 7 if true:

import pifaceio
pf = pifaceio.PiFace()
...
# Fetch inputs (i.e. single byte)
pf.read()
first_two_inputs_on = pf.read_pin(0) and pf.read_pin(1)

# Now write that state to output pin 7
pf.write_pin(7, first_two_inputs_on)

# Do final (actual) write when all output pin states are set.
pf.write()

Simulated "interrupt" processing example by light-weight poll every 10 msecs:

import pifaceio, time
pf = pifaceio.PiFace()

def process_change():
    'On any changed inputs, read inputs and write outputs'
    pf.write_pin(7, pf.read_pin(0) and pf.read_pin(1))

    # .. etc .. do logic using pf.read_pin() and pf.write_pin()

# Loop forever polling inputs ..
last = None
while True:
    data = pf.read()

    # Do processing only on change
    if last != data:
        last = data
        process_change()
        pf.write()        # note write() only writes if output changes

    time.sleep(.01)

PIFACE PACKAGE BACKWARDS COMPATIBILITY

The following piface API will work compatibly, but performance is slightly degraded compared to reading and writing the single input and output bytes using the canonical new and preferred pifaceio API described above. However, performance is still significantly superior compared to using the original piface package itself.

#import piface.pfio as pf (change this to next line)
import pifaceio as pf

# The following calls should be approximately compatible:
pf.init()
value = pf.digital_read(pin)
pf.digital_write(pin, value)
pf.deinit()

You can also use multiple boards with this compatibility interface, e.g. as follows where board can be from 0 to 7.

value = pf.digital_read(pin, board)
pf.digital_write(pin, value, board)

UPGRADE

cd pifaceio  # source dir, as above
git pull
sudo python ./setup.py install

LICENSE

Copyright (C) 2013 Mark Blakeney. This program is distributed under the terms of the GNU General Public License. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License at http://www.gnu.org/licenses/ for more details.

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