Overview

Raspberry Pis are the poster child of micro computers and are extremely popular and available from multiple resellers. They are very well supported. The following functionality is supported:

  • 2x I²C buses
  • 2x SPI bus
  • 46x high performance edge detection enabled memory-mapped GPIO pins

Support

Drivers

Configuration

The pins function can be affected by device overlays as defined in /boot/config.txt. The full documentation of overlays is at boot/overlays/README. Documentation for the file format is on raspberry pi site

GPIO

All GPIO are supported at extremely high speed via memory mapped GPIO registers, with full interrupt based edge detection.

I²C

The BCM238x has two I²C buses but it is recommended to only use the second. Enabling /dev/i2c-1 permanently:

sudo raspi-config nonint do_i2c 0

Speed

The speed default is 62.5kHz, not 100kHz as advertized by the driver. The I²C spec calls for 400kHz for many I²C devices so it is worth increasing the default bus speed. For a bandwidth heavy device like an SSD1306, this may be a requirement.

Warning: A slower speed is selected and used than the one specified. Here’s a few values tested:

RequestedActual
100kHz62.5kHz
400kHz250Khz
500kHz312Khz
600kHz375Khz
650kHz410Khz
1000kHz625Khz

It doesn’t make any sense. The author is not sure why asking for 400Khz doesn’t lead to 375kHz, looks like the i2c_bcm2708 driver speed selection algorithm is poorly implemented. As such, this is currently (as of 2017-04-12) recommended to ask for 600kHz to get 375kHz.

If the value was overriden, it can be queried with:

cat /sys/module/i2c_bcm2708/parameters/baudrate

Writing to this file doesn’t update the bus speed.

Permanent

The I²C #1 bus speed can be increased permanently to 375kHz by either:

  • Append to /boot/config.txt: dtparam=i2c_baudrate=600000
  • Create a file in /etc/modprobe.d/ (e.g. /etc/modprobe.d/i2c.conf) containing: options i2c_bcm2708 baudrate=600000

Refer to the official documentation for more information.

Temporary

The I²C bus speed can be changed temporarily until next reboot by running:

sudo modprobe -r i2c_bcm2708 && sudo modprobe i2c_bcm2708 baudrate=600000

Either of the 3 methods above affect both buses I2C0 and I2C1 simultaneously.

PWM

To take back control of the PWM pins to use as general purpose PWM, comment out the following line in /boot/config.txt:

dtparam=audio=on

SPI

The BCM238x has 3 SPI buses but only 2 are soldered on the RPi.

The first SPI controller (/dev/spidev0.0 and /dev/spidev0.1) can be enabled permanently with:

sudo raspi-config nonint do_spi 0

Second SPI

If you need two SPI buses simultaneously, /dev/spidev1.0 can be enabled with adding to /boot/config.txt:

dtoverlay=spi1-1cs

On rPi3, bluetooth must be disabled with:

dtoverlay=pi3-disable-bt

and bluetooth UART service needs to be disabled with:

sudo systemctl disable hciuart

SPI buffer size

SPI transaction size is limited by spidev driver buffer size. Its default is 4096 bytes. The current value can be viewed with the following command:

cat /sys/module/spidev/parameters/bufsiz

In some case this may not be enough like with the FLIR Lepton. The permanent fix is to edit /boot/cmdline.txt to insert spidev.bufsiz=65536 at the beginning of the line.

I2S

I2S can be enabled by adding the following in /boot/config:

dtparam=i2s=on

IR

IR/Hardware

Raspbian has a specific device tree overlay named lirc-rpi to enable hardware based decoding of IR signals. This loads a kernel module that exposes itself at /dev/lirc0. Enable permanently by adding to /boot/config.txt:

dtoverlay=lirc-rpi,gpio_out_pin=5,gpio_in_pin=13,gpio_in_pull=high

Default pins 17 and 18 clashes with SPI1 so change the pin if you plan to enable both SPI buses.

See boot/overlays/README for more details on configuring the kernel module.

IR/Software

Once the kernel module is configured, you need to point lircd to it. Run the following as root to edit /etc/lirc/hardware.conf to point lircd to use lirc_rpi kernel module:

sed -i s'/DRIVER="UNCONFIGURED"/DRIVER="default"/' /etc/lirc/hardware.conf
sed -i s'/DEVICE=""/DEVICE="\/dev\/lirc0"/' /etc/lirc/hardware.conf
sed -i s'/MODULES=""/MODULES="lirc_rpi"/' /etc/lirc/hardware.conf

IR/References

Linux lirc_rpi.c

Someone made a version that supports multiple devices: github.com/bengtmartensson/lirc_rpi

UART

Kernel boot messages go to the UART (0 or 1, depending on Pi version) at 115200 bauds.

On Rasberry Pi 1 and 2, UART0 is used.

On Raspberry Pi 3, UART0 is connected to bluetooth so the console is connected to UART1 instead. Disabling bluetooth also reverts to use UART0 and not UART1.

UART0 can be disabled with:

dtparam=uart0=off

UART1 can be enabled with:

dtoverlay=uart1

Buying

As the most popular micro-computer, Raspberry Pis can be bought at many places.

The periph authors do not endorse any specific seller. These are only provided for your convenience.