SPI

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:


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:


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{
           compatible = "rockchip,spidev";
           status = "okay";
           reg = <0x0>;
           spi-max-frequency = <50000000>;
       };
   };

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

FAQs

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.