reading from spi

spi_reading.c

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/spi/spidev.h>

/*
	To read data from an SPI (Serial Peripheral Interface) device in Linux, you can use the spidev driver. Here are the basic steps:

	Open the spidev device file using the open system call. The device file is usually located at /dev/spidevX.Y, where X is the SPI bus number and Y is the chip select (slave) number.

	Set the SPI mode, which determines the clock polarity and phase, using the ioctl system call. The mode is specified as a bit field with the following options:

	SPI_CPOL: Clock polarity (0 or 1)
	SPI_CPHA: Clock phase (0 or 1)
	SPI_MODE_0: CPOL=0, CPHA=0
	SPI_MODE_1: CPOL=0, CPHA=1
	SPI_MODE_2: CPOL=1, CPHA=0
	SPI_MODE_3: CPOL=1, CPHA=1
	Set the maximum transfer speed using the SPI_IOC_WR_MAX_SPEED_HZ ioctl. This determines the maximum clock frequency that the SPI bus will use.

	Use the read system call to read data from the device. The read call takes a buffer and a length as arguments and returns the number of bytes read.

	Here is some sample code to demonstrate the basic process:
 */
void spi() {
	uint8_t tx[] = { 0x01, 0x02, 0x03, 0x04 };
	uint8_t rx[4] = { 0 };
	struct spi_ioc_transfer tr = {
		.tx_buf = (unsigned long) tx, // mosi
		.rx_buf = (unsigned long) rx, // miso
		.len = sizeof(tx),
		.delay_usecs = 0,
		.speed_hz = 200000000,
		.bits_per_word = 8,
	};

	int fd = open("/dev/spidev1.0", O_RDWR);
	if (fd < 0) {
		perror("open");
		exit(1);
	}

	// Set SPI mode to mode 0
	uint8_t mode = SPI_MODE_0;
	if (ioctl(fd, SPI_IOC_WR_MODE, &mode) < 0) {
		perror("ioctl");
		exit(1);
	}

	// Set maximum transfer speed
	uint32_t speed = 200000000; // 200 MHz
	if (ioctl(fd, SPI_IOC_WR_MAX_SPEED_HZ, &speed) < 0) {
		perror("ioctl");
		exit(1);
	}

	uint32_t bits = 8;
	if (ioctl(fd, SPI_IOC_WR_BITS_PER_WORD, &bits) < 0) {
		perror("ioctl");
		exit(1);
	}

	// Read data from SPI device
	if (ioctl(fd, SPI_IOC_MESSAGE(1), &tr) < 0) {
		perror("ioctl");
		exit(1);
	}

	// Print received data
	printf("Received: ");
	for (int i = 0; i < sizeof(rx); i++) {
		printf(" %02X", rx[i]);
	}
	printf("\n");

	close(fd);
}