RPiShift

A python module for interfacing a 595 shift register with the Raspberry Pi

Installation

You can install RPiShift one of two ways:

• Using pip:

$ pip install RPiShift

• Using setup.py and setuptools:

$ git clone git@github.com:kneitinger/RPiShift.git
$ ./setup.py install RPiShift

Usage

Hardware

To use RPiShift attach a 595 chip as indicated by the following schematic

Software

Initializing the module

If the 595 is only device attached to the GPIO pins of the Raspberry Pi, begin by importing the RPiShift module

import RPiShift

After that call the shiftr initialization function to configure the pins:

# Argument list
# Shifter(dataPin, clockPin, latchPin, [chainLength], [boardMode])
# Args in [] are optional and default to 1 and GPIO.BOARD respectively
shift = RPiShift.Shifter(11,13,15)

NOTE: When using the RPi.GPIO library with other buttons, motors, etc. at the same time as the RPiShift library, be sure to pass the board mode (GPIO.BCM or GPIO.BOARD) to ensure that there are not conflicting pin mappings.

Writing bytes

To write a byte to the shift register simply call the writeByte method with the desired value to write:

# Turn on all LEDs (with only one 595 in the chain)
shift.writeByte(0xFF)
# Turn off all LEDs
shift.writeByte(0x00)
# Display a binary 37 (00100101)
shift.writeByte(37)
# Display every 4 LEDs in a 4-chain circuit
shift.writeByte(0x0F0F0F0F)

Currently there is no bounds checking, so be sure you're writing the correct amount of digits for the length of your chain: 2 hex digits or 8 binary digits per 595.

Writing single pins

To write a value to a single pin, call the writePin method with the desired pin (beginning at 0) and value (0 or 1):

# Blink each pin in order
for i in range (8*shift.CHAIN):
    shift.writePin(i,1)
    time.sleep(.125)
    shift.writePin(i,0)

Toggling a pin

To toggle the state of a single pin, call the togglePin method with the desired pin to change

# Turn on every other pin
shift.writeByte(0x55)
# Toggle 0b01010101 to 0b10101010
while True:
    for i in range(8):
        shift.togglePin(i)
        time.sleep(.125)

Pushing single bits ala FIFO queue

With the pushBit and writeLatch functions, you can treat the 595(s) as a FIFO queue, pushing single bits into the register and triggering the update with writeLatch. For example, we can push a 1, write the latch, push a 0 seven times, and get a scrolling effect:

for i in range(64):
    shift.pushBit(((i+1)%8)==0)
    shift.writeLatch()
    time.sleep(.1)

Cleanup practices

It is a good idea to call the cleanup function at the end of the program, but there are two special cases to watch for.

• Keyboard Interrupts To ensure that keyboard interrupts are taken care of gracefully, put your main function in a try: block and cleanup the pins if the KeyboardInterrupt is raised:

try:
    mainFunc()
except KeyboardInterrupt:
    shift.cleanup()
    exit(1)

• When used with RPi.GPIO or other output libraries If you are using another GPIO module such as RPi.GPIO, wiringpi, etc.; be mindful that calling cleanup may interact with the output of other modules. While this should not be an issue if called at the end of the program, care should be taken in regards to calling cleanup mid-program.

Further info

For more information, check out the example programs and the source code. If you have any questions, comments or feedback feel free to email me at kylejkneitinger@gmail.com. Happy hacking!