SPI

This page will summarize the informations to use the SPI bus on APF boards.

Overview

SPI (Serial Peripheral Interface) is a 4-wire full-duplex serial bus. Wires used in SPI are:

• MOSI, Master Output Slave Input : send data to slave.
• MISO, Master Input Slave Output : receive data from slave.
• SCLK, : Serial Clock signal used to synchronise the transmission. (In imx27 (apf27) maximum frequency of SCLK is 22.167MHz in master mode and 16.625 in slave mode.
• SSx, Slave Select : used to select the slave for communication with master.

On APF27, the i.MX27 contains 3 SPI devices that can be configured in master (slave mode is not supported by Linux). APF9328 (i.MXL) contains 2 SPI.

Linux configuration

APF27Dev with 2.6.29 kernel

SPI is used by some kernel drivers; when selecting them it will naturally activate the corresponding SPI bus. To use the SPI from user space you have first to be sure that the corresponding SPI bus is activated:

$ make linux-menuconfig

    Device Drivers  --->
        [*] SPI support  --->
            ...
            <*>   Freescale iMX SPI controller 
            [*]     CSPI1
            [*]     CSPI2
            [*]     CSPI3
            ...

And user SPI device interface (spidev) option must be selected too (here as a module):

    Device Drivers  --->
        [*] SPI support  --->
            ...
            *** SPI Protocol Masters ***
            <M>   User mode SPI device driver support
            ...

With 2.6.29 kernels, on APF27Dev, spidev device uses the bus number 2 and PortB17 as chip select (J22 connector, muxed with USB signals). So /dev/spidev1.2 has to be used to access your device from userspace. To change this (for expert), the structure spi_board_info must be modified in platform file apf27-dev.c (in directory buildroot/output/build/linux-2.6.29.6/arch/arm/mach-mx2/) :

#ifdef CONFIG_SPI_SPIDEV
	{
		.modalias		= "spidev",
		.controller_data	= &spidev_hw, /* for chip select */
		.max_speed_hz		= 8000000, /* 8MHz */
		.bus_num		= 1, /* SPI2 */
		.mode			= SPI_MODE_1,
		.chip_select		= 2,
		.platform_data		= &apf27_spidev_config,
	},
#endif /* CONFIG_SPI_SPIDEV */
...
#ifdef CONFIG_SPI_SPIDEV
#define SPIDEV_CS (GPIO_PORTB | 17)
...

APF27Dev with 3.19+ kernels

• SPI3 bus (CS0), available on J8 connector, has been configured for spidev. /dev/spidev2.0 is the device to use with these kernels. You'll need an up-to-date apf27.dtb
• to change it you will have to edit arch/arm/boot/dts/imx27-apf27dev.dts in Linux sources and then recompile kernel (make linux) and update dtb from U-Boot (run update_dtb).

APF6Dev with 4.1+-legacy kernels

• SPI1 bus (CS0), available on J5 connector, has been configured for spidev. /dev/spidev0.0 is the device to use with these kernels.

For more details on spidev usage, see kernel documentation.

Usage

When loaded, this driver will create a /dev/spidevN.0 node to read/write on SPI bus from userspace (N depends on your platform).

# modprobe spidev
# ls /dev/spidev*

Linux user space C code

To write/read on SPI via spidev driver from your program, use ioctl as described in spidev documentation.

Linux kernel provides a test tool, to compile and use it:

$ mkdir spitest
$ cd spitest
$ cp ../buildroot/output/build/linux-x.x.x/Documentation/spi/spidev_test.c .
 or
$ cp ./buildroot/output/build/linux-4.x.x/tools/spi/spidev_test.c .    (on recent kernels)
$ ../buildroot/output/host/usr/bin/arm-linux-gcc -o spidev_test spidev_test.c
$ scp spidev_test root@192.168.1.22:/root/

# ./spidev_test -D /dev/spidev2.0 -s 8000000

A module named as_spi has also been written in AsDevices to ease spidev usage from userspace.

Tested SPI chips

• ADC : Max1027
• EEPROM : 93LCXX
• FLIR Lepton

Links

• SPI description on wikipedia.
• Linux spidev documentation