一、 串口API

在Linux系统中,操作设备的统一接口就是:open/ioctl/read/write。
对于UART,又在ioctl之上封装了很多函数,主要是用来设置行规程。
所以对于UART,编程的套路就是:

• open
• 设置行规程,比如波特率、数据位、停止位、检验位、RAW模式、一有数据就返回
• read/write
  怎么设置行规程?行规程的参数用结构体termios来表示,可以参考Linux串口—struct termios结构体

typedef unsigned char 	cc_t;
typedef unsigned int 	speed_t;
typedef unsgined int 	tcflag_t;

#define NCCS 19
struct termios {
	tcflag_t c_iflag;		/* input mode flags */
	tcflag_t c_oflag;		/* output mode flags */
	tcflag_t c_cflag;		/* control mode flags */
	tcflag_t c_lflag;		/* local mode flags */
	cc_t c_line;			/* line discipline */
	cc_t c_cc[NCCS];		/* control characters */
};

这些函数在名称上有一些惯例：

• tc: terminal control
• cf: control flag

函数不多,主要是需要设置好termios中的参数,这些参数很复杂,可以参考Linux串口—struct termios结构体。

二、编程

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <termios.h>
#include <stdlib.h>

int set_opt(int fd, int nSpeed, int nBits, char nEvent, int nStop)
{
	struct termios newtio, oldtio;
	
	if(tcgetattr(fd, &oldtio) != 0) {
		perror("SetupSerial 1");
		return -1;
	}

	bzero(&newtio, sizeof(newtio));
	newtio.c_cflag |= CLOCAL|CREAD;
	newtio.c_cflag &= ~CSIZE;

	newtio.c_lflag &= ~(ICANON|ECHO|ECHOE|ISIG);
	newtio.c_oflag &= ~OPOST;

	switch(nBits) {
	case 7:
		newtio.c_cflag |= CS7;
		break;
	case 8:
		newtio.c_cflag |= CS8;
		break;
	}

	switch(nEvent) {
	case 0:
		newtio.c_cflag |= PARENB;
		newtio.c_cflag |= PARODD;
		newtio.c_iflag |= (INPCK | ISTRIP);
		break;
	case 'E':
		newtio.c_iflag |= (INPCK|ISTRIP);
		newtio.c_cflag |= PARENB;
		newtio.c_cflag &= ~PARODD;
		break;
	case 'N':
		newtio.c_cflag &= ~PARENB;
		break;
	}

	switch(nSpeed) {
	case 2400:
		cfsetispeed(&newtio, B2400);
		cfsetospeed(&newtio, B2400);
		break;
	case 4800:
		cfsetispeed(&newtio, B4800);
		cfsetospeed(&newtio, B4800);
		break;
	case 9600:
		cfsetispeed(&newtio, B9600);
		cfsetospeed(&newtio, B9600);
		break;
	case 115200:
		cfsetispeed(&newtio, B115200);
		cfsetospeed(&newtio, B115200);
		break;
	default:
		cfsetispeed(&newtio, B9600);
		cfsetospeed(&newtio, B9600);
		break;
	}


	if(nStop == 1)
		newtio.c_cflag &= ~CSTOPB;
	else if(nStop == 2)
		newtio.c_cflag |= CSTOPB;

	newtio.c_cc[VMIN] = 1;
	newtio.c_cc[VTIME] = 0;

	tcflush(fd, TCIFLUSH);
	if((tcsetattr(fd, TCSANOW, &newtio)) != 0) {
		perror("com set error");
		return -1;
	}

	return 0;
}

int open_port(char *com)
{
	int fd;
	fd = open(com, O_RDWR|O_NOCTTY);
	if(-1 == fd) {
		return -1;
	}

	if(fcntl(fd, F_SETFL, 0) < 0) {
		printf("fcntl failed\n");
		return -1;
	}
	return fd;
}

int main(int argc, char *argv[])
{
	int fd;
	int iRet;
	char c;

	if(argc != 2) {
		printf("Usage: \n");
		printf("%s </dev/ttySAC1 or other>\n", argv[0]);
		return -1;
	}

	fd = open_port(argv[1]);
	if(fd < 0) {
		printf("open %s err!\n", argv[1]);
		return -1;
	}

	iRet = set_opt(fd, 115200, 8, 'N', 1);
	if(iRet) {
		printf("set port err!\n");
		return -1;
	}

	printf("Enter a char: ");
	while(1) {
		scanf("%c", &c);
		iRet = write(fd, &c, 1);
		iRet = read(fd, &c, 1);
		if(iRet == 1) {
			printf("get: %02x %c\n", c, c);
		} else {
			printf("can not get data\n");
		}
	}

	return 0;
}

三、上机实验

短接串口的RX和TX

root@npi:~/test# ./a.out /dev/ttymxc2 
Enter a cahr: a
get: 61 a
get: 0a 


get: 0a 

 
get: 0a 

a 
get: 61 a
get: 0a