M5Stack_Linux_Libs/doc/example_doc/linux_spi.md

## SPI
Serial Peripheral Interface (SPI) is a common serial communication protocol used for high-speed data transfer between external devices such as microcontrollers, sensors, and storage devices.

Here are some important concepts about SPI that beginners need to understand:

1. **Serial Communication:** SPI is a serial communication protocol that allows devices to transmit multiple bits of data at the same time, enabling high-speed data transfer.

2. **Master-Slave Structure:** SPI communication involves two types of devices: master and slave. The master device generates the clock signal and controls the communication, while the slave device responds to the master device's commands.

3. **Communication Lines:** SPI uses multiple lines for communication:
   - SCLK (Serial Clock): The clock signal generated by the master device for synchronizing data transfer.
   - MOSI (Master Out Slave In): The line for sending data from the master device to the slave device.
   - MISO (Master In Slave Out): The line for sending data from the slave device to the master device.
   - SS/CS (Slave Select/Chip Select): Used for selecting the slave device's line, allowing communication between multiple devices.

4. **Clock Synchronization:** Data transfer in SPI communication is synchronous, meaning data is transmitted based on the clock signal.

5. **Frame Format:** SPI communication consists of a series of data frames. Data transfer usually occurs at each clock cycle, enabling high-speed transmission.

6. **Speed:** SPI communication can be very fast because data transfer is synchronous. The speed can be set according to requirements.

7. **Full Duplex:** SPI is a full-duplex communication protocol, allowing simultaneous bidirectional data transfer.

8. **Applications:** SPI is widely used in embedded systems, storage devices, sensors, displays, communication devices, and other fields.

examples:

```c
//-----------------------------------------------------------------------------
#include "linux_spi/linux_spi.h"
#include <stdio.h>
//-----------------------------------------------------------------------------
#define SPI_DEVICE "/dev/spidev0.0"
//-----------------------------------------------------------------------------
int main()
{
  spi_t spi;
  char buf[1024];
  int i;

  int retv = spi_init(&spi,
                      SPI_DEVICE, // filename like "/dev/spidev0.0"
                      0,          // SPI_* (look "linux/spi/spidev.h")
                      0,          // bits per word (usually 8)
                      2500000);   // max speed [Hz]

  printf(">>> spi_init() return %d\n", retv);

  retv = spi_read(&spi, buf, 1024);
  printf(">>> spi_read(1024) return %d\n", retv);

  for (i = 0; i < 1024; i++)
    buf[i] = 0x55;

  retv = spi_write(&spi, buf, 1024);
  printf(">>> spi_write(1024) return %d\n", retv);

  spi_free(&spi);

  return 0;
}

```

When compiling the above program, you can compile and run it in the examples/linux_spi directory of [M5Stack_Linux_Libs]().  
Enter the Linux device:
``` bash
# Clone repository
git clone https://github.com/M5STACK/M5Stack_Linux_Libs.git

# Enter directory
cd M5Stack_Linux_Libs/examples/linux_spi

# Compile
scons 

```