Developing IO Access in Applications

This page only gives some hints about io access.

RS232/RS485

In typical Linux style, serial ports are represented by files within the operating system. These files usually pop-up in /dev/, and begin with the name tty*.

Common names are:

Example:

int serial_port = open("/dev/ttymxc3 ", O_RDWR);

// Check for errors
if (serial_port < 0) {
    printf("Error %i from open: %s\n", errno, strerror(errno));
}

unsigned char msg[] = { 'H', 'e', 'l', 'l', 'o', '\r' };
write(serial_port, msg, sizeof(msg));

close(serial_port);

CAN

The Linux C/C API allows you to control a SocketCAN interface via a C/C application.

SocketCAN supports standard frame format (SFF), extended frame format (EFF) and CAN FD frames.

Example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/socket.h>

#include <linux/can.h>
#include <linux/can/raw.h>

int main(int argc, char **argv)
{
	int s; 
	struct sockaddr_can addr;
	struct ifreq ifr;
	struct can_frame frame;

	printf("CAN Sockets Demo\r\n");

	if ((s = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
		perror("Socket");
		return 1;
	}

	strcpy(ifr.ifr_name, "can0" );
	ioctl(s, SIOCGIFINDEX, &ifr);

	memset(&addr, 0, sizeof(addr));
	addr.can_family = AF_CAN;
	addr.can_ifindex = ifr.ifr_ifindex;

	if (bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
		perror("Bind");
		return 1;
	}

	frame.can_id = 0x555;
	frame.can_dlc = 5;
	sprintf(frame.data, "Hello");

	if (write(s, &frame, sizeof(struct can_frame)) != sizeof(struct can_frame)) {
		perror("Write");
		return 1;
	}

	if (close(s) < 0) {
		perror("Close");
		return 1;
	}

	return 0;
}

Ethernet

For ethernet commmunication the Linux networking socket layer user interface is used.

The BSD compatible sockets are the uniform interface between the user process and the network protocol stacks in the kernel.

The protocol modules are grouped into protocol families such as AF_INET, AF_IPX, and AF_PACKET, and socket types such as SOCK_STREAM or SOCK_DGRAM.

The usually interface names begin with the name eth*.

GPIO

The Linux kernel includes a user space API based on character devices for managing and controlling GPIOs ( General-Purpose Input/Output). 

Common gpio's are:

CP 3x

WP 4x

SOM imx6ul EvalBoard

SOM imx8mm EvalBoard

Example:

#include <linux/gpio.h>
 
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <stdint.h>
#include <stdlib.h>
 
#define DEV_NAME "/dev/gpiochip0"
 
int fd, ret;
fd = open(DEV_NAME, O_RDONLY);
if (fd < 0)
{
    printf("Unabled to open %s: %s", dev_name, strerror(errno));
    return;
}
/*
    control GPIO here, such as:
    - configure
    - read
    - write
    - polling
*/
(void)close(fd)

ADC/AIN

Analog input, also known as Analog to Digital Converter (ADC), is a hardware peripheral that can read an analog voltage value and convert it to a digital value that can be used by the processor.

For Linux the ADC subsystem is abstracted through the sysfs interface.

SOM imx6ul EvalBoard

I2C

Todo

SPI

Tod