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Serial Peripheral Interface (SPI)

SPI is a full-duplex serial communication protocol that is commonly used for inter-chip communication such as the interface between a microcontroller and flash memory. In this tutorial we will use GreatFET One to interface with a strip of RGB LEDs.

Many RGB LED strips (such as DotStar strips from Adafruit) are made of APA102 LEDs that use the SPI communication protocol. You can use jumper wires to connect a strip to the following pins on GreatFET One:

  • clock: J1 pin 38 (SCK)
  • data: J1 pin 39 (MOSI)
  • VCC (3.3 V supply): J1 pin 2
  • GND (ground): J1 pin 1

Open an interactive Python shell:

gf shell

Turn on the first LED to test communication:

gf.spi.transmit([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff])

Turn it off:

gf.spi.transmit([0x00, 0x00, 0x00, 0x00, 0xff, 0x00, 0x00, 0x00])

The spi.transmit() method executes a bi-directional data transfer. The GreatFET One acts as SPI master, pulling down SSEL (slave select) to activate the slave and then driving SCK (SPI clock) and MOSI (master out, slave in). We haven't wired up SSEL because the APA102 LEDs do not require it.

As it clocks data out on MOSI, it also clocks in the same amount of data from the slave on MISO (master in, slave out). The argument passed to spi.transmit() is an array of bytes to transmit. The method returns the received data as an array of bytes. The APA102 does not implement bi-directional communication, so we haven't connected anything to MISO. That means that spi.transmit() always returns an array of bytes with all bits set (0xff) every time we call it, but we can ignore that.

You can paste the following program into your shell to see a fancy light show!

import bitstring
import time
import math

def led_frame(red=0, green=0, blue=0, brightness=0x1f):
    frame = bitstring.Bits(length=3, uint=0x7)
    frame += bitstring.Bits(length=5, uint=(brightness & 0x1f))
    frame += bitstring.Bits(length=8, uint=(blue & 0xff))
    frame += bitstring.Bits(length=8, uint=(green & 0xff))
    frame += bitstring.Bits(length=8, uint=(red & 0xff))
    return frame

def chase():
    brightness = 1
    length = 60 # set this to the number of LEDs in your strip
    period = length / 2.0
    start_frame = bitstring.Bits('0x00000000')
    pixels = bitstring.BitStream()
    for i in range(length):
        red = int(127+127*math.cos(2*math.pi*(i/period)))
        green = int(127+127*math.cos(2*math.pi*((i+period/3)/period)))
        blue = int(127+127*math.cos(2*math.pi*((i+2*period/3)/period)))
        pixels += led_frame(red, green, blue, brightness)
    while(True):
        gf.spi.transmit((start_frame + pixels).bytes)
        time.sleep(0.02)
        pixels.ror(32) # rotate the pixel array by 32 bits which is one LED

chase()