/dev/ttyUSB延迟15ms

异步串行的一个字节需要<0.1ms,在115200波特的线上。(10位周期:开始+8+停止。)

以下C程序:

#define _POSIX_C_SOURCE 199309L /* needed for CLOCK_MONOTONIC */
#include <termios.h> /* tcattr */
#include <unistd.h>
#include <string.h> /* strerror */
#include <errno.h> /* errno */
#include <fcntl.h> /* O_NDELAY, O_RDWR, O_EXCL */
#include <stdio.h> /* printf */
#include <stdint.h>
#include <stdlib.h> /* exit */
#include <inttypes.h> /* PRIu64 */
#include <time.h> /* struct timespec */

int open_ttyusb(uint8_t n)
{
    char path[] = "/dev/ttyUSBnnn"; /* long enough for all values of uint8_t */
    sprintf(path, "/dev/ttyUSB%i", n);
    int fd = open(path, O_NDELAY|O_RDWR|O_EXCL);

    if (fd < 0) {
        exit(-1);
    }

    struct termios t;

    tcgetattr(fd, &t);             /* save old one  */
    t.c_iflag = t.c_oflag = 0;
    t.c_cflag = B115200;

    t.c_cflag |= (CLOCAL | CREAD); /* always 8-bit and local (no modem controls) */
    t.c_cflag |= CS8;              /* 8 bit character size mask */

    t.c_iflag = IGNBRK;

    t.c_lflag = 0;
    t.c_cc[VMIN] = 1;
    t.c_cc[VTIME] = 0;
    tcsetattr(fd, TCSAFLUSH, &t); /* do it all and flush input */
    tcflush(fd, TCIFLUSH);

    uint8_t rubbish[4096];
    while (read(fd, rubbish, 4096) > 0); /* make sure that there is nothing left */

    return fd;
}

uint64_t now(void) {
    struct timespec spec;

    clock_gettime(CLOCK_MONOTONIC, &spec);
    return spec.tv_nsec + spec.tv_sec * 1000000000;
} 

int main(void) {
    int fd0 = open_ttyusb(0);
    int fd1 = open_ttyusb(1);

    for (int i = 0; i < 10; ++i) { /* repeat the test 10 times */
        uint64_t start = now();
        char tx = 'A' + i; /* send chars 'A' through 'J' */
        write(fd0, &tx, 1); 
        uint8_t rx;
        while (read(fd1, &rx, 1) < 1); /* loop until read succeeds */
        if (rx != 'A' + i) { /* check that we've read the right char */
            exit(-2);
        }
        uint64_t elapsed = now() - start;
        printf("time_taken: %"PRIu64".%"PRIu64"ms\n", elapsed / 1000000, elapsed % 1000000);
    }   
}

输出:

time_taken: 1.292915ms
time_taken: 15.881472ms
time_taken: 15.859872ms
time_taken: 15.926402ms
time_taken: 15.975379ms
time_taken: 15.851588ms
time_taken: 15.882685ms
time_taken: 15.944355ms
time_taken: 15.915532ms
time_taken: 15.925496ms

(每次跑步的第一次拍摄时间为0-16ms。)

我有两个USB FTDI适配器,通过RX和TX交叉连接在一起。

• 为什么字节从一个ttyUSB到另一个需要15毫秒?
• 为什么第一次传输通常要快得多?
• 可以更改代码以减少延迟吗?