Serial Peripheral Interface (SPI)

Raspberry Pi computers are equipped with a number of SPI buses. SPI can be used to connect a wide variety of peripherals - displays, network controllers (Ethernet, CAN bus), UARTs, etc. These devices are best supported by kernel device drivers, but the spidev API allows userspace drivers to be written in a wide array of languages.

SPI Hardware

Rapsberry Pi Zero, 1, 2 and 3 have three SPI controllers:

• SPI0, with two hardware chip selects, is available on the header of all Pis; there is also an alternate mapping that is only available on compute modules.
• SPI1, with three hardware chip selects, is available on all Raspberry Pi models except the original Pi 1 models A and B.
• SPI2, also with three hardware chip selects, is only available on Compute Module 1, 3 and 3+.

On the Raspberry Pi 4, 400 and Compute Module 4 there are four additional SPI buses: SPI3 to SPI6, each with 2 hardware chip selects. These extra SPI buses are available via alternate function assignments on certain GPIO pins - see the BCM2711 ARM Peripherals datasheet.

Chapter 10 in the BCM2835 ARM Peripherals datasheet describes the main controller. Chapter 2.3 describes the auxiliary controller.

Pin/GPIO mappings

SPI0

SPI0 alternate mapping (compute modules only, except CM4)

SPI1

SPI2 (compute modules only, except CM4)

SPI3 (BCM2711 only)

SPI4 (BCM2711 only)

SPI5 (BCM2711 only)

SPI6 (BCM2711 only)

Master modes

Signal name abbreviations

SCLK - Serial CLocK
CE   - Chip Enable (often called Chip Select)
MOSI - Master Out Slave In
MISO - Master In Slave Out
MOMI - Master Out Master In

Standard mode

In Standard SPI mode the peripheral implements the standard 3 wire serial protocol (SCLK, MOSI and MISO).

Bidirectional mode

In bidirectional SPI mode the same SPI standard is implemented, except that a single wire is used for data (MOMI) instead of the two used in standard mode (MISO and MOSI). In this mode, the MOSI pin serves as MOMI pin.

LoSSI mode (Low Speed Serial Interface)

The LoSSI standard allows issuing of commands to peripherals (LCD) and to transfer data to and from them. LoSSI commands and parameters are 8 bits long, but an extra bit is used to indicate whether the byte is a command or parameter/data. This extra bit is set high for a data and low for a command. The resulting 9-bit value is serialized to the output. LoSSI is commonly used with MIPI DBI type C compatible LCD controllers.

Transfer modes

• Polled
• Interrupt
• DMA

Speed

The CDIV (Clock Divider) field of the CLK register sets the SPI clock speed:

SCLK = Core Clock / CDIV

If CDIV is set to 0, the divisor is 65536. The divisor must be a multiple of 2, with odd numbers rounded down. Note that not all possible clock rates are usable because of analogue electrical issues (rise times, drive strengths, etc).

See the Linux driver section for more info.

Chip Selects

Setup and hold times related to the automatic assertion and de-assertion of the CS lines when operating in DMA mode are as follows:

• The CS line will be asserted at least 3 core clock cycles before the msb of the first byte of the transfer.
• The CS line will be de-asserted no earlier than 1 core clock cycle after the trailing edge of the final clock pulse.

SPI Software

Linux driver

The default Linux driver is spi-bcm2835.

SPI0 is disabled by default. To enable it, use raspi-config, or ensure the line dtparam=spi=on is not commented out in /boot/config.txt. By default it uses 2 chip select lines, but this can be reduced to 1 using dtoverlay=spi0-1cs. dtoverlay=spi0-2cs also exists, and without any parameters it is equivalent to dtparam=spi=on.

To enable SPI1, you can use 1, 2 or 3 chip select lines, adding in each case:

dtoverlay=spi1-1cs  #1 chip select
dtoverlay=spi1-2cs  #2 chip select
dtoverlay=spi1-3cs  #3 chip select

to the /boot/config.txt file. Similar overlays exist for SPI2, SPI3, SPI4, SPI5 and SPI6.

The driver does not make use of the hardware chip select lines because of some limitations - instead it can use an arbitrary number of GPIOs as software/GPIO chip selects. This means you are free to choose any spare GPIO as a CS line, and all of these SPI overlays include that control - see /boot/overlays/README for details, or run (for example) dtoverlay -h spi0-2cs (dtoverlay -a | grep spi might be helpful to list them all).

Speed

The driver supports all speeds which are even integer divisors of the core clock, although as said above not all of these speeds will support data transfer due to limits in the GPIOs and in the devices attached. As a rule of thumb, anything over 50MHz is unlikely to work, but your mileage may vary.

Supported Mode bits

SPI_CPOL    - Clock polarity
SPI_CPHA    - Clock phase
SPI_CS_HIGH - Chip Select active high
SPI_NO_CS   - 1 device per bus, no Chip Select
SPI_3WIRE   - Bidirectional mode, data in and out pin shared

Bidirectional or "3-wire" mode is supported by the spi-bcm2835 kernel module. Please note that in this mode, either the tx or rx field of the spi_transfer struct must be a NULL pointer, since only half-duplex communication is possible. Otherwise, the transfer will fail. The spidev_test.c source code does not consider this correctly, and therefore does not work at all in 3-wire mode.

Supported bits per word

• 8 - Normal
• 9 - This is supported using LoSSI mode.

Transfer modes

Interrupt mode is supported on all SPI buses. SPI0, and SPI3-6 also support DMA transfers.

SPI driver latency

This thread discusses latency problems.

spidev

spidev presents an ioctl-based userspace interface to individual SPI CS lines. Device Tree is used to indicate whether a CS line is going to be driven by a kernel driver module or managed by spidev on behalf of the user; it is not possible to do both at the same time. Note that Raspberry Pi’s own kernels are more relaxed about the use of Device Tree to enable spidev - the upstream kernels print warnings about such usage, and ultimately may prevent it altogether.

Using spidev from C

There is a loopback test program in the Linux documentation that can be used as a starting point. See the Troubleshooting section.

Using spidev from Python

There are several Python libraries that provide access to spidev, including spidev (pip install spidev - see https://pypi.org/project/spidev/) and SPI-Py (https://github.com/lthiery/SPI-Py).

Using spidev from a shell such as bash

# Write binary 1, 2 and 3
echo -ne "\x01\x02\x03" > /dev/spidev0.0

Other SPI libraries

There are other userspace libraries that provide SPI control by directly manipulating the hardware: this is not recommended.

Troubleshooting

Loopback test

This can be used to test SPI send and receive. Put a wire between MOSI and MISO. It does not test CE0 and CE1.

wget https://raw.githubusercontent.com/raspberrypi/linux/rpi-3.10.y/Documentation/spi/spidev_test.c
gcc -o spidev_test spidev_test.c
./spidev_test -D /dev/spidev0.0
spi mode: 0
bits per word: 8
max speed: 500000 Hz (500 KHz)

FF FF FF FF FF FF
40 00 00 00 00 95
FF FF FF FF FF FF
FF FF FF FF FF FF
FF FF FF FF FF FF
DE AD BE EF BA AD
F0 0D

Some of the content above has been copied from the elinux SPI page, which also borrows from here. Both are covered by the CC-SA license.