Controlling an SPI device with the Raspberry Pi

The Raspberry Pi has a Broadcom BCM 2835 chip allowing it to interface with SPI devices on its GPIO pins. There are two chip select pins meaning that the Pi can control two devices simultaneously.

P1 Header Pin	Function
19	MOSI – master output slave input
21	MISO – master input slave output
23	SCLK – clock
24	CE0 – chip enable 0
26	CE1 – chip enable 1

Step 1: Enable SPI on the Raspberry Pi

1. In your Pi’s terminal, run

   sudo raspi-config

2. Go to Advanced Options > SPI
3. Choose “Yes” for both questions then select Finish to exit raspi-config
4. Either reboot your Pi or run this command to load the kernel module

   sudo modprobe spi-bcm2708

Step 2: Install spidev

Spidev is a python module that allows us to interface with the Pi’s SPI bus.Watch movie online The Transporter Refueled (2015)

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install python-dev python3-dev
cd ~
git clone https://github.com/doceme/py-spidev.git
cd py-spidev
make
sudo make install

Step 3: Python script

Finally, we can write and run a python script to control the SPI device.

1. Create a file called spi-test.py in your favorite editor

   #!/usr/bin/python
   
   import spidev
   import time
   
   spi = spidev.SpiDev()
   spi.open(0, 0)
   spi.max_speed_hz = 7629
   
   # Split an integer input into a two byte array to send via SPI
   def write_pot(input):
       msb = input >> 8
       lsb = input & 0xFF
       spi.xfer([msb, lsb])
   
   # Repeatedly switch a MCP4151 digital pot off then on
   while True:
       write_pot(0x1FF)
       time.sleep(0.5)
       write_pot(0x00)
       time.sleep(0.5)

2. Make the file executable and run it

   chmod +x spi-test.py
   sudo ./spi-test.py

Notes on spidev

Unless the spi.max_speed_hz field is a value accepted by the driver, the script will fail when you run it. The field can be set to these values on the raspberry pi:

Speed	spi.max_speed_hz value
125.0 MHz	125000000
62.5 MHz	62500000
31.2 MHz	31200000
15.6 MHz	15600000
7.8 MHz	7800000
3.9 MHz	3900000
1953 kHz	1953000
976 kHz	976000
488 kHz	488000
244 kHz	244000
122 kHz	122000
61 kHz	61000
30.5 kHz	30500
15.2 kHz	15200
7629 Hz	7629

Two SPI devices can be controlled in python by creating two SpiDev objects, one for each device.

spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 976000

spi2 = spidev.SpiDev()
spi2.open(0, 1)
spi2.max_speed_hz = 976000