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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 (COPI)
  • 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 controller, pulling down CS (chip select) to activate the peripheral and then driving SCK (SPI clock) and COPI (controller out, peripheral in). We haven't wired up CS because the APA102 LEDs do not require it.

As it clocks data out on COPI, it also clocks in the same amount of data from the peripheral on CIPO (controller in, peripheral 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 CIPO. 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()

SPI Pin Naming

Older GreatFET hardware and documentation used pin names MOSI, MISO, and SSEL which have been deprecated in favor of COPI, CIPO, and CS. Related documentation such as the LPC4330 user manual may continue to use the deprecated names. See OSHWA's Resolution to Redefine SPI Signal Names for more information.