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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 official pifacedigitalio Python package for the PiFace Digital board. In my simple benchmark of polled reads and writes (see next section), pifaceio performs an order of magnitude 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 version 3.4 and later.

Performance Benchmarks Against pifacedigitalio

A small benchmark program is included in the repository. It produces the following sample of simple polled read and write performance comparisons between pifacedigitalio and pifaceio on a Raspberry Pi2B.

Function pifacedigitalio pifaceio Speedup
Input reads per second 1740 16317 x 9.4
Output writes per second 884 11590 x 13.1
Input reads per second (classic API) 1773 11128 x 6.3
Output reads per second (classic API) 889 9140 x 10.3

INSTALLATION

The pifaceio pypi package is available from PyPi so you can install it using pip. If pip is not already installed run:

sudo apt-get install python3-pip

Then use pip to install the pifaceio package:

sudo pip3 install -U pifaceio

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

sudo pifaceio-install-spidev.sh

UPGRADE

sudo pip3 install -U pifaceio

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

As an alternative API, pifaceio also implements the "classic" API from the pifacedigitalio and it's predecessor piface packages. The classic 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 to the original pifacedigitalio and piface packages as shown in the above performance comparison section.

#import piface.pfio as pf (change this line ..)
#import pifacedigitalio as pf (or this line .., to the following 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)

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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Provides a Python interface to the PiFace Digital peripheral board for the Raspberry Pi.

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