How to use the SPI ports on Aria SOM

This article illustrates how to use the SPI on Aria SOM

The Serial Peripheral Interface (SPI) is a synchronous serial communication interface specification used for short-distance communication, primarily in embedded systems. The interface was developed by Motorola in the mid-1980s and has become a de facto standard. Typical applications include Secure Digital cards, memory, converter and liquid crystal displays.

Wikipedia definition

Pinout of SPI ports

The SPI bus lines are located on the following pins:

Signal	Dir	Description	Aria pins	MCU line
SPI0 MOSI	O	Master Output Slave Input	S11	PA12
SPI0 MISO	I	Master Input Slave Output	S12	PA11
SPI0 SCLK	O	Clock	S10	PA13
SPI0 CS0	O	Chip Select 0	S9	PA14
SPI0 CS1	O	Chip Select 1	S16	PA7

Not all the signales are available by default and not all SPI lines are listed on this table or muxed in this way.

Refer to the MCU datasheets to find other lines.

Microchip AT91SAM9G25 CPU datasheet

Enable the Linux Kernel SPI modules

Check if your Kernel has the SPI drivers already active by typing:

ls /dev/spi*

If the files do not exist this tutorial:

Compiling Linux Kernel 4.19.128 LTS for Aria

to know how to cross compile the Linux Kernel and how to configure the drivers using the device tree to enable the lines you need.

Enable the SPIDEV user interface and the Microchip SPI device driver as shown below using the make ARCH=arm menuconfig comand:

Device Drivers  --->
  [*] SPI support  ---> 
    <*>   Atmel SPI Controller 
    <*>   User mode SPI device driver support

then configure the device tree (arch/arm/boot/dts/acme-aria.dts) with this definition inside the section ahb and apb:

spi0: spi@f0000000 {
    status = "okay";
    interrupts = <13 4 5>;
    cs-gpios = <&pioA 14 0>, <&pioA 7 0>;

    device@0 {
        compatible = "spidev";
        spi-max-frequency = <5000000>;
        reg = <0>;
    };

    device@1 {
        compatible = "spidev";
        spi-max-frequency = <5000000>;
        reg = <1>;
    };
};

Copy the zImage and acme-aria.dtb file inside the first microSD partition, boot and check if any /dev/i2c exists.

ls /dev/spi*
/dev/spidev0.0  /dev/spidev0.1

Check the system log

When you reboot the board you should see a message like this:

[    0.820312] atmel_spi f0000000.spi: version: 0x212
[    0.820312] atmel_spi f0000000.spi: Using dma0chan2 (tx) and dma0chan3 (rx) for DMA transfers
[    0.828125] atmel_spi f0000000.spi: Atmel SPI Controller at 0xf0000000 (irq 29)

by typing the command:

dmesg

Use the SPI from user space

This is an example on how to use the SPI interface from user space using the device file /dev/spidev32766.0 created by the spidev driver.

We will use a Microchip SPI DAC model MCP4822

wired to the Aria G25 SOM in this way:

| MCP4822 | ARIA   | SAM9G20      |
|---------|--------|--------------|
| 1 VDD   | 3V3    |              |
| 2 CS    | S9     | PA14 - NPCS0 |
| 3 SCK   | S10    | PA13 - SPCK  |
| 4 SDI   | S11    | PA12 - MOSI  |
| 5 LDAC  | GND    |              |
| 6 VOUTB | OUT B  |              |
| 7 VSS   | GND    |              |
| 8 VOUTA | OUT A  |              |

with this C program we can set the VOUT A and VOUT B voltage in a range od 0 to 2 volt.