Tag Archives: spidev

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