How to use SPI from Linux userland with spidev

1. Purpose

This article describes how to use SPI. The SPI overview is described in SPI overview article.

2. SPI Loop between MOSI and MISO

Short-circuit the MISO and MOSI lines of SPI bus to create a loopback allows the bus to receive the same data it is sending. This is interesting to execute quickly basic tests and to execute performance tests.

2.1. Board connections

A wire has to be added to connect MISO and MOSI pins (D12 et D11 pins on STM32MP157X-DKX ARDUINO connector)

2.2. DT configuration (board level)

Board DT configuration has to be updated to:

• configure cs-gpios and activate spi4 node
• add spidev configuration to spi4 node

&spi4 {
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&spi4_pins_a>;
	pinctrl-1 = <&spi4_sleep_pins_a>;
	status = "okay";
	cs-gpios = <&gpioe 11 0>;

	spidev@0{
		compatible = "spidev";
		reg = <0>;
		spi-max-frequency = <4000000>;
		#address-cells = <1>;
		#size-cells = <0>;
	};
};

2.3. Simple data transfer

 echo -ne "\xAA\xBB" > /dev/spidev0.0
 more /dev/spidev0.0

2.4. Data transfer with spidev_test

The spidev_test tool, from the Linux^® kernel, provides a test tool for spidev.

2.4.1. Source code location

The Linux^® kernel spidev_test tool source code can be found under tools/spi.

2.4.2. spidev_test compilation

This tool is not compiled by default when compiling the Linux kernel for the target board. It can be compiled independently: How to build Linux kernel user space tools and then installed on the target.

2.4.3. List the spidev options

The spidev_test tool options can be listed:

 spidev_test -h
Usage: spidev_test [-DsbdlHOLC3vpNR24SItx]
  -D --device   device to use (default /dev/spidev1.1)
  -s --speed    max speed (Hz)
  -d --delay    delay (usec)
  -b --bpw      bits per word
  -i --input    input data from a file (e.g. "test.bin")
  -o --output   output data to a file (e.g. "results.bin")
  -l --loop     loopback
  -H --cpha     clock phase
  -O --cpol     clock polarity
  -L --lsb      least significant bit first
  -C --cs-high  chip select active high
  -3 --3wire    SI/SO signals shared
  -v --verbose  Verbose (show tx buffer)
  -p            Send data (e.g. "1234\xde\xad")
  -N --no-cs    no chip select
  -R --ready    slave pulls low to pause
  -2 --dual     dual transfer
  -4 --quad     quad transfer
  -S --size     transfer size
  -I --iter     iterations
  -t --txonly   simplex tx transfer
  -r --rxonly   simplex rx transfer

2.4.4. Example to transfer 32 bytes in full-duplex

 spidev_test -D /dev/spidev0.0 -v
			spi mode: 0x0
			bits per word: 8
			max speed: 500000 Hz (500 KHz)
			TX | 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 F0 0D  | ......@.... .................. .
			RX | 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 F0 0D  | ......@.... .................. .

3. References

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Serial Peripheral Interface

Device Tree

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