在嵌入式应用领域中,串口是最为常见的一种硬件通信接口。因为其具备协议简单,硬件电路精简等优势使得串口基本成为MCU、计算机或嵌入式产品的标配接口。本文仅介绍在Linux系统下串口编程需要使用的API和一些应用技巧,关于串口的背景知识介绍,以及Windows系统下串口编程读者可以移步至其他文章。

Linux系统下串口的操作主要分为如下部分:

  • 串口打开、关闭
  • 串口参数设置
  • 串口数据发送与接收
  • 串口MODEM信号设置与读取
  • 串口Break信号发送

 

可以熟练掌握并应用以上串口功能已经可以应对Linux系统上串口应用的大多数场景了,针对更高级的串口用法可以阅读《Linux串口编程-进阶篇》,包含Linux系统使用非标准波特率、同步等待Modem信号变化、串口参数VTIME和VMIN的作用、RS485串口功能开关等。为方便用户使用我们将以上串口操作均封装成了独立的函数,可以极大的节约开发时间。

1、串口打开

    /**
     * libtty_open - open tty device
     * @devname: the device name to open
     *
     * In this demo device is opened blocked, you could modify it at will.
     */
    static int libtty_open(const char *devname)
    {
    	int fd = open(devname, O_RDWR | O_NOCTTY | O_NDELAY);
    	int flags = 0;
     
    	if (fd < 0) {
    		perror("open device failed");
    		return -1;
    	}
        /* 恢复串口为阻塞状态 */
    	flags = fcntl(fd, F_GETFL, 0);
    	flags &= ~O_NONBLOCK;
    	if (fcntl(fd, F_SETFL, flags) < 0) {
    		printf("fcntl failed.\n");
    		return -1;
    	}
        /* 测试该设备是否为tty设备 */
    	if (isatty(fd) == 0) {
    		printf("not tty device.\n");
    		return -1;
    	} else
    		printf("tty device test ok.\n");
     
    	return fd;
    }

 

Note:

  • devname 参数为设备绝对路径,如:“/dev/ttyUSB0”
  • O_NOCTTY标志用于通知系统,这个程序不会成为对应这个设备的控制终端。如果没有指定这个标志,那么任何一个输入(如SIGINT等)都将会影响用户的进程;
  • O_NDELAY标志与O_NONBLOCK 等效,但这里不仅仅是设置为非阻塞,还用于通知系统,这个程序不关心 DCD 信号线所处的状态(即与设备相连的另一端是否激活或者停止)。如果用户指定了这一标志,则进程将会一直处在休眠状态,直到 DCD 信号线被激活;

2、串口关闭

    /**
     * libtty_close - close tty device
     * @fd: the device handle
     *
     * The function return 0 if success, others if fail.
     */
    static int libtty_close(int fd)
    {
    	return close(fd);
    }

3、串口设置

    /**
     * libtty_setopt - config tty device
     * @fd: device handle
     * @speed: baud rate to set
     * @databits: data bits to set
     * @stopbits: stop bits to set
     * @parity: parity to set
     * @hardflow: hardflow to set
     *
     * The function return 0 if success, or -1 if fail.
     */
    static int libtty_setopt(int fd, int speed, int databits, int stopbits, char parity, char hardflow)
    {
    	struct termios newtio;
    	struct termios oldtio;
    	int i;
     
    	bzero(&newtio, sizeof(newtio));
    	bzero(&oldtio, sizeof(oldtio));
     
        /* 先保存之前配置,以防后续步骤出错无法恢复 */
    	if (tcgetattr(fd, &oldtio) != 0) {
    		perror("tcgetattr");
    		return -1;
    	}
    	newtio.c_cflag |= CLOCAL | CREAD;
    	newtio.c_cflag &= ~CSIZE;
     
        /* 串口波特率设置*/
        switch (speed): {
        case 1200:
            cfsetspeed(&newtio, B1200);
            break;
        case 2400:
            cfsetspeed(&newtio, B2400);
            break;
        case 4800:
            cfsetspeed(&newtio, B4800);
            break;
        case 9600:
            cfsetspeed(&newtio, B9600);
            break;
        case 19200:
            cfsetspeed(&newtio, B19200);
            break;
        case 38400:
            cfsetspeed(&newtio, B38400);
            break;
        case 57600:
            cfsetspeed(&newtio, B57600);
            break;
        case 115200:
            cfsetspeed(&newtio, B115200);
            break;
        case 230400:
            cfsetspeed(&newtio, B230400);
            break;
        case 460800:
            cfsetspeed(&newtio, B460800);
            break;
        case 921600:
            cfsetspeed(&newtio, B921600);
            break;
        default:
            break;
        }
    	for (i = 0; i < sizeof(speed_arr) / sizeof(int); i++) {
    		if (speed == name_arr[i]) {      
    			cfsetispeed(&newtio, speed_arr[i]); 
    			cfsetospeed(&newtio, speed_arr[i]);   
    		} 
    	}
    	/* 数据位设置 */
    	switch (databits) {
    	case 5:
    		newtio.c_cflag |= CS5;
    		break;
    	case 6:
    		newtio.c_cflag |= CS6;
    		break;
    	case 7:
    		newtio.c_cflag |= CS7;
    		break;
    	case 8:
    		newtio.c_cflag |= CS8;
    		break;
    	default:
    		fprintf(stderr, "unsupported data size\n");
    		return -1;
    	}
     
    	/* 校验位设置 */
    	switch (parity) {
    	case 'n':
    	case 'N':
    		newtio.c_cflag &= ~PARENB;    /* Clear parity enable */
    		newtio.c_iflag &= ~INPCK;     /* Disable input parity check */
    		break;
    	case 'o':
    	case 'O':
    		newtio.c_cflag |= (PARODD | PARENB); /* Odd parity instead of even */
    		newtio.c_iflag |= INPCK;     /* Enable input parity check */
    		break;
    	case 'e':
    	case 'E':
    		newtio.c_cflag |= PARENB;    /* Enable parity */
    		newtio.c_cflag &= ~PARODD;   /* Even parity instead of odd */
    		newtio.c_iflag |= INPCK;     /* Enable input parity check */
    		break;
    	case 'm':
    	case 'M':
    		newtio.c_cflag |= PARENB;    /* Enable parity */
    		newtio.c_cflag |= CMSPAR;    /* Stick parity instead */
    		newtio.c_cflag |= PARODD;    /* Even parity instead of odd */
    		newtio.c_iflag |= INPCK;     /* Enable input parity check */
    		break;
    	case 's':
    	case 'S':
    		newtio.c_cflag |= PARENB;    /* Enable parity */
    		newtio.c_cflag |= CMSPAR;    /* Stick parity instead */
    		newtio.c_cflag &= ~PARODD;   /* Even parity instead of odd */
    		newtio.c_iflag |= INPCK;     /* Enable input parity check */
    		break;
    	default:
    		fprintf(stderr, "unsupported parity\n");
    		return -1;
    	}
     
    	/* 停止位设置 */
    	switch (stopbits) {
    	case 1:
    		newtio.c_cflag &= ~CSTOPB;
    		break;
    	case 2:
    		newtio.c_cflag |= CSTOPB;
    		break;
    	default:
    		perror("unsupported stop bits\n");
    		return -1;
    	}
     
        /* 硬件流控设置 */
    	if (hardflow)
    		newtio.c_cflag |= CRTSCTS;
    	else
    		newtio.c_cflag &= ~CRTSCTS;
     
        /* 串口读操作参数设置 */
    	newtio.c_cc[VTIME] = 10;	/* Time-out value (tenths of a second) [!ICANON]. */
    	newtio.c_cc[VMIN] = 0;	/* Minimum number of bytes read at once [!ICANON]. */
     
        /* 刷新串口缓冲区 */
    	tcflush(fd, TCIOFLUSH);
     
        /* 设置 */
    	if (tcsetattr(fd, TCSANOW, &newtio) != 0) {
    		perror("tcsetattr");
    		return -1;
    	}
     
    	return 0;
    }

常规串口参数的设置均可以通过如上函数进行设定,注释比较详细。

包括串口波特率、数据位、停止位、硬件流控设置等。

4、串口发送

    /**
     * libtty_write - write data to uart
     * @fd: file descriptor of tty device
     * @buf: buffer to write
     * @count: write length
     *
     * The function return the number of bytes written if success, others if fail.
     */
    static int libtty_write(int fd, char *buf, int count)
    {
        return write(fd, buf, count);
    }

5、串口读取

    /**
     * libtty_read - read data from uart
     * @fd: file descriptor of tty device
     * @buf: pointer to read buffer
     * count: read length
     *
     * The function return the number of bytes read if success, others if fail.
     */
    static int libtty_read(int fd, char *buf, int count)
    {
    	return read(fd, buf, count);
    }

6、串口MODEM设置

    /**
     * libtty_tiocmset - modem set
     * @fd: file descriptor of tty device
     * @bDTR: 0 on inactive, other on DTR active
     * @bRTS: 0 on inactive, other on RTS active
     *
     * The function return 0 if success, others if fail.
     */
    static int libtty_tiocmset(int fd, char bDTR, char bRTS)
    {
    	unsigned long controlbits = 0;
     
    	if (bDTR)
    		controlbits |= TIOCM_DTR;
    	if (bRTS)
    		controlbits |= TIOCM_RTS;
     
    	return ioctl(fd, TIOCMSET, &controlbits);
    }

MODEM输出信号包括DTR和RTS信号,这2个信号可以由串口应用进行控制,常用于下载或IO控制等。

针对TTL/CMOS串口,DTR和RTS无效时为高电平,有效时为高电平。

7、串口MODEM读取

    /**
     * libtty_tiocmget - modem get
     * @fd: file descriptor of tty device
     * @modembits: pointer to modem status
     *
     * The function return 0 if success, others if fail.
     */
    static int libtty_tiocmget(int fd, unsigned long *modembits)
    {
    	int ret;
     
    	ret = ioctl(fd, TIOCMGET, modembits);
    	if (ret == 0) {
    		if (*modembits & TIOCM_DSR)
    			printf("DSR Active!\n");
    		if (*modembits & TIOCM_CTS)
    			printf("CTS Active!\n");
    		if (*modembits & TIOCM_CD)
    			printf("DCD Active!\n");
    		if (*modembits & TIOCM_RI)
    			printf("RI Active!\n");
    	}
     
    	return ret;
    }

MODEM输出信号包括DSR、CTS、DCD和RI信号,这4个信号可以由串口应用主动读取其有效状态,常用于状态指示或同步等。

针对TTL/CMOS串口,DSR、CTS、DCD和RI无效时为高电平,有效时为高电平。

8、串口Break信号

    /**
     * libtty_sendbreak - uart send break
     * @fd: file descriptor of tty device
     *
     * Description:
     *  tcsendbreak() transmits a continuous stream of zero-valued bits for a specific duration, if the  termi©\
     *	nal is using asynchronous serial data transmission.  If duration is zero, it transmits zero-valued bits
     *	for at least 0.25 seconds, and not more that 0.5 seconds.  If duration is not zero, it sends  zero-val©\
     *	ued bits for some implementation-defined length of time.
     *
     *  If  the terminal is not using asynchronous serial data transmission, tcsendbreak() returns without tak©\
     *	ing any action.
     */
    static int libtty_sendbreak(int fd)
    {
    	return tcsendbreak(fd, 0);
    }

串口Break信号在一些特定场景下会使用到,针对TTL/CMOS串口而言,串口Break是指串口的TXD保持一定时间的低电平。常见于一些实验仪器需要使用Break信号作为有效的开始信号。需要注意:tcsendbreak的第二个参数填0,表示串口TXD持续低电平0.25s~0.5s,如果参数为非0值,则持续参数中指定的时间。

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