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9. SPI

9.1. Introduction​

SPI is a high-speed, full-duplex, synchronous serial communication interface for connecting microcontrollers, sensors, storage devices, etc. The AIO-3576JQ development board provides the SPI interface, and the specific position is as follows:

_images/usage_spi_interface.jpg

9.2. How SPI works​

SPI works in a master-slave mode, which typically has one master device and one or more slave devices, requiring at least four wires, respectively:

CS		    slice selection signal
SCLK		clock letter
MOSI		master device data output and slave device data input
MISO		master device data input and slave device data output

The Linux kernel uses a combination of CPOL and CPHA to represent the four working modes of the current SPI:

CPOL＝0，CPHA＝0		SPI_MODE_0
CPOL＝0，CPHA＝1		SPI_MODE_1
CPOL＝1，CPHA＝0		SPI_MODE_2
CPOL＝1，CPHA＝1		SPI_MODE_3
  • CPOL : Represents the state of the initial level of the clock signal, 0 is the low level and 1 is the high level.
  • CPHA : Is sampling along which clock, 0 is sampling along the first clock and 1 is sampling along the second clock.

The waveforms of SPI’s four working modes are as follows:

_images/usage_spi_waveform_en.jpg

9.3. Interface usage​

Linux provides a SPI user interface with limited functionality. If IRQ or other kernel driver interfaces are not required, consider using spidev interface to write user-level programs to control SPI devices. The corresponding path in the AIO-3576JQ development board is /dev/spidev1.0.

spidev corresponding driver code is kernel-5.10/drivers/spi/spidev.c.

The config in the kernel needs to select SPI_SPIDEV:

 │ Symbol: SPI_SPIDEV [=y]
 │ Type  : tristate
 │ Prompt: User mode SPI device driver support
 │   Location:
 │     -> Device Drivers
 │       -> SPI support (SPI [=y])
 │   Defined at drivers/spi/Kconfig:684
 │   Depends on: SPI [=y] && SPI_MASTER [=y]

DTS configuration like follows:

&spix{
    status = "okay";
    max-freq = <50000000>;
    spidev1: spidev@00&#123;
        compatible = "rockchip,spidev";
        status = "okay";
        reg = <0x0>;
        spi-max-frequency = <50000000>;
    &#125;;
&#125;;

Please refer to kernel-5.10/Documentation/spi/spidev.rst for detailed instructions.

9.4. FAQs​

9.4.1. Q1: SPI data transfer exception?​

A1 : Make sure the IOMUX configuration of SPI 4 pins is correct. Confirm that when TX sends data, TX pins have normal waveform, CLK frequency is correct, CS signal is pulled down, and mode matches the device.