linux串口通信编程

在linux下, 串口也被当做一个文件来使用, 所以串口传输数据实际上也就是对文件进行read/write操作.

UART串口基本知识:

一般, 串口至少有三根线:

地线GND, 接收线RX和发送线TX. 有的开发板还可能把控制线CTS/RTS也引出来.

串口查看方式:

在PC端, 如果用USB口连接, 一般显示为/dev/ttyUSBx, 其中x为0, 1, 2...

在开发板上,一般显示为/dev/ttySx, 其中x为0, 1, 2...

在电脑上可以使用下面的方法查看或者设置串口属性:

查看串口所有属性: sudo stty -F /dev/ttyUSB1 -a

设置串口波特率: sudo stty -F /dev/ttyUSB1 ispeed 1152000 ospeed 1152000

串口常用参数:

波特率: 每秒钟传送的bit数. --> 通信速度.

数据位: 每字节中实际所占的bit数, 取决于通信协议的选取. 比如，标准的ASCII码是0～127（7位）。扩展的ASCII码是0～255（8位）。

停止位: 单个数据包的最后一位. 典型值为1, 1.5和2位..

奇偶校验位: 有四种检错方式, 偶、奇、高和低。可以没有校验位.

比特率: 数字信号的传输速率，单位时间内传输的二进制代码的有效位(bit)数，其单位为bit/s(bps)、Kbps或Mbps (此处K和M分别为1000和1000000)。

串口编程流程:

打开串口 --> 设置串口参数 --> 读写数据 --> 关闭串口

其中设置串口属性是比较重要的环节.

下面代码的功能: 给串口发送一个启动命令, 然后进入循环---从串口接收数据(数据头和数据分开传输), 并发送命令消息, 直到遇到终端信号或程序出错.

第一个是C语言实现，第二个是python实现。

#include <unistd.h>
#include <signal.h>
#include <time.h>
#include <fcntl.h>
#include <string.h>
#include <getopt.h>
#include <errno.h>
#include <termios.h>
#include <stdio.h>
#include <stdlib.h>

#define UART_NAME "/dev/ttyS0"

static int g_quit;
static int g_speed;

static void handle_signal(int sig)
{
  if (SIGINT == sig || SIGTERM == sig)
    g_quit = 1;
}

static struct sigaction sigact = {
  .sa_handler = handle_signal,
};

static struct option longopts[] = {
  {"help", no_argument, NULL, 'h'},
  {"baudrate", required_argument, NULL, 'r'},
  {0, 0, 0, 0},
};


static void print_usage(void)
{
  fprintf(stdout, "usage:\n");
  fprintf(stdout, "\t-r | --baudrate  the serial baud rate\n");
  fprintf(stdout, "\t-h | --help      show this help and exit\n");
}


static int parse_arg(int argc, char **argv)
{
  int ret = 0;
  char c = '0';

  while (
    (c = getopt_long(argc, argv, "-:r:h", longopts, NULL))
    != -1) {
    switch (c) {
    case 'r':
      g_speed = atoi(optarg);
      break;
    case 'h':
      ret = -1;
      break;
    case ':':
      fprintf(stdout, "%c require argument\n", optopt);
      ret = -1;
      break;
    case '?':
      fprintf(stdout, "%c invalid argument\n", optopt);
      ret = -1;
      break;
    default:
      break;
    }
  }

  return ret;
}

static int uart_open(char *filename)
{
  int fd;

  //fd = open(filename, O_RDWR);
  fd = open(filename, O_RDWR|O_NOCTTY|O_NDELAY);
  if (fd < 0) {
    printf("%s %d: failed to open output file\n",
        __FILE__, __LINE__);
    return -1;
  }

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

static int uart_set(int fd, int speed, int flow_ctrl,
    int databits, int parity, int stopbits)
{
  struct termios options;
  int ret = 0;

  if (tcgetattr(fd, &options) != 0) {
    printf("Setup serial fialed!\n");
    return -1;
  }

  int i;
  int speed_arr[] = {B1500000, B1152000, B1000000, B921600,
    B576000, B500000, B460800, B230400, B115200, B57600,
    B38400, B19200, B9600, B4800, B2400, B1800, B1200,
    B600, B300, B200, B150, B134, B110, B75, B50, B0};
  int name_arr[] = {1500000, 1152000, 1000000, 921600,
    576000, 500000, 460800, 230400, 115200, 57600,
    38400, 19200, 9600, 4800, 2400, 1800, 1200,
    600, 300, 200, 150, 134, 110, 75, 50, 0};

  if (tcgetattr(fd, &options) != 0) {
    printf("Setup serial fialed!\n");
    return -1;
  }

  printf("speed = %d\n", speed);
  for (i = 0; i < sizeof(speed_arr) / sizeof(int); i++) {
    if (speed == name_arr[i]) {
      ret = cfsetispeed(&options, speed_arr[i]);
      if (ret) {
        perror("cfsetispeed");
        printf("set in speed failed\n");
      }

      ret = cfsetospeed(&options, speed_arr[i]);
      if (ret) {
        perror("cfsetispeed");
        printf("set out speed failed\n");
      }
      break;
    }
  }

  options.c_cflag |= CLOCAL;
  options.c_cflag |= CREAD;

  switch (flow_ctrl) {
  case 0: // no flow control
    options.c_cflag &= ~CRTSCTS;
    break;
  case 1: // hardware flow control
    options.c_cflag |= CRTSCTS;
    break;
  case 2: //software flow control
    options.c_cflag |= IXON|IXOFF|IXANY;
    break;
  default:
    printf("Unsupported flow control\n");
    return -1;
  }

  options.c_cflag &= ~CSIZE;
  switch (databits) {
  case 5:
    options.c_cflag |= CS5;
    break;
  case 6:
    options.c_cflag |= CS6;
    break;
  case 7:
    options.c_cflag |= CS7;
    break;
  case 8:
    options.c_cflag |= CS8;
    break;
  default:
    printf("Unsupported databits!\n");
    return -1;
  }

  switch (parity) {
  case 'n': //no parity
  case 'N':
    options.c_cflag &= ~PARENB;
    options.c_iflag &= ~INPCK;
    break;
  case 'o': //odd parity
  case 'O':
    options.c_cflag |= (PARODD | PARENB);
    options.c_iflag &= INPCK;
    break;
  case 'e': //even parity
  case 'E':
    options.c_cflag |= PARENB;
    options.c_cflag &= ~PARODD;
    options.c_iflag |= INPCK;
    break;
  case 's': //blank
  case 'S':
    options.c_cflag &= ~PARENB;
    options.c_iflag &= ~CSTOPB;
    break;
  default:
    printf("Unsupported parity\n");
    return -1;
  }

  switch (stopbits) {
  case 1:
    options.c_cflag &= ~CSTOPB;
    break;
  case 2:
    options.c_cflag |= CSTOPB;
    break;
  default:
    printf("Unsupported stop bits\n");
    return -1;
  }

  //mode
  options.c_lflag  &= ~(ICANON | ECHO | ECHOE | ISIG);  /*Input*/
  options.c_oflag  &= ~OPOST;   /*Output*/

  //wait_time: 0.1s; min char to read:1
  options.c_cc[VTIME] = 1;
  options.c_cc[VMIN] = 1;

  //if data overflow, receive data, but not read
  tcflush(fd, TCIFLUSH);

  //save configuration
  if (tcsetattr(fd, TCSANOW, &options) != 0) {
    printf("set serial error!\n");
    return -1;
  }
  return 0;
}

static int recv_data(int fd, int fdtest)
{
  int inflg = 0;
  int ret, size;
  int *data = NULL;

  do {
    if (g_quit)
      break;

    inflg = 0;
    ret = 0;
    size = 0;
    int hsize = read(fd, &size, 4);

    if (hsize < 0) {
      printf("read header size error\n");
      ret = -1;
      break;
    }

    if (size <= 0)
      continue;

    printf("size = %d\n", size);
    if (data) {
      free(data);
      data = NULL;
    }
    data = calloc(1, size);
    if (!data) {
      printf("calloc failed\n");
      ret = -1;
      break;
    }

    int len = 0;
    int left_size = size - len;

    do {
      len = read(fd, data, left_size);
      if (len < 0) {
        printf("read fd error\n");
        ret = -1;
        break;
      }
      //printf("real read size = %d\n", len);
      int n = write(fdtest, data, len);

      if (n < 0) {
        printf("write error!\n");
        ret = -1;
        break;
      }
      left_size = left_size - len;
    } while (left_size > 0);

    inflg = 1;
  } while (!inflg);

  if (data) {
    free(data);
    data = NULL;
  }
  return ret;
}

int main(int argc, char **argv)
{
  int ret = 0;
  int fd, fdtest;
  char send[1] = {'a'};

  ret = sigaction(SIGINT, &sigact, NULL);
  ret |= sigaction(SIGTERM, &sigact, NULL);
  if (ret) {
    printf("%s line%d: %s\n",
      __FILE__, __LINE__, strerror(errno));
    return -1;
  }

  g_speed = 9600;

  ret = parse_arg(argc, argv);
  if (ret) {
    print_usage();
    return -1;
  }

  fd = uart_open("/dev/ttyS0");
  if (fd < 0)
    return -1;

  int speed = g_speed;
  int flow_ctrl = 0;
  int databits = 8;
  int stopbits = 1;
  int parity = 'O';

  ret = uart_set(fd, speed, flow_ctrl, databits, parity, stopbits);
  if (ret) {
    printf("uart_set failed\n");
    goto out;
  }

  fdtest = open("/mnt/recv.h264", O_RDWR|O_SYNC);
  if (fdtest < 0) {
    printf("open fdtest failed\n");
    goto out;
  }

  int n = write(fd, send, 1);

  if (n <= 0) {
    printf("send a failed\n");
    close(fdtest);
    goto out;
  }

  do {
    if (g_quit)
      break;

    ret = recv_data(fd, fdtest);

    usleep(1000*10);
    send[0] = 'b';

    int res = write(fd, send, 1);

    if (res < 0)
      break;
    /* printf("%s: %s: %d\n", __FILE__, __func__, __LINE__); */
  } while (!ret);
  close(fdtest);
out:
  close(fd);
  return ret;
}

python

#!/usr/bin/python
import serial
import time
import struct

def w():
  baud = 115200
  fd = open("/home/sarah/2newh264/test15.h264", "w+")
  ser = serial.Serial('/dev/ttyUSB1', baud, timeout=8)
  print "baud: ", baud
  cmd0 = 'a'
  cmd1 = 'b'
  ser.write(cmd0)
  print "send \""+cmd0+"\" to remotes"
  while (1):
    print "send \""+cmd1+"\" to remotes"
    h = ser.read(4)
    if not h:
      continue
    size = struct.unpack("i", h)
    print "size: ", size[0]
    input = ser.read(size[0])
    fd.write(input)
    print "\n"
    ser.write(cmd1)
  ser.close()
  close(fd)

w()