SAMA5D3开发板USB设备读写：数据读取方法与写入验证问询

Hey there! Let's work through your two questions about reading from and verifying writes to your USB-connected device on the SAMA5D3 board. First, let's break down the issues in your current code and fix them step by step.

Your current reading logic has a couple of key issues that are preventing you from getting data properly:

• Uninitialized/incorrect termios configuration: The speed variable isn't assigned a value, and setting c_iflag/c_oflag to speed doesn't make sense. For custom baud rates with BOTHER, you need to set c_ispeed and c_ospeed directly.
• Timing issue with FIONREAD: You're checking for available bytes immediately after writing, but the device might not have sent a response yet—especially in non-blocking mode.
• Non-blocking mode pitfalls: Using O_NONBLOCK means read will return immediately if no data is ready, which is why you're hitting the "error" case often.

Here's how to fix this:

Step 1: Fix the Termios Configuration #

First, properly set up your serial port parameters (adjust baud rate to match your device):

struct termios usb_parm;
// Initialize with current port settings
tcgetattr(fd1, &usb_parm);

// Reset flags and set custom baud rate with BOTHER
usb_parm.c_cflag &= ~CBAUD;
usb_parm.c_cflag |= BOTHER;
usb_parm.c_ispeed = 9600; // Set your desired baud rate here
usb_parm.c_ospeed = 9600;

// Configure for raw mode (tweak based on your device's requirements)
usb_parm.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
usb_parm.c_oflag &= ~OPOST;
usb_parm.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
usb_parm.c_cflag &= ~(CSIZE | PARENB);
usb_parm.c_cflag |= CS8; // 8 data bits

// Apply settings immediately
retval = tcsetattr(fd1, TCSANOW, &usb_parm);
if (retval != 0) {
    perror("Failed to set serial attributes");
    close(fd1);
    return -1;
}

Step 2: Improve the Reading Logic #

Instead of checking FIONREAD once, use select() to wait for data to arrive (this handles timing properly, whether you use blocking or non-blocking mode):

char read_buff[64] = {0}; // Fixed-size buffer with room for null terminator
int bytes_read = 0;
fd_set readfds;
struct timeval tv;

// Wait up to 2 seconds for a response
tv.tv_sec = 2;
tv.tv_usec = 0;
FD_ZERO(&readfds);
FD_SET(fd1, &readfds);

int select_ret = select(fd1 + 1, &readfds, NULL, NULL, &tv);
if (select_ret > 0 && FD_ISSET(fd1, &readfds)) {
    int byteavailable = 0;
    ioctl(fd1, FIONREAD, &byteavailable);
    if (byteavailable > 0) {
        bytes_read = read(fd1, read_buff, sizeof(read_buff) - 1); // Leave space for null terminator
        if (bytes_read > 0) {
            read_buff[bytes_read] = '\0'; // Ensure valid string
            printf("Read buff=%s\n", read_buff);
            printf("Bytes read=%d\n", bytes_read);
        } else {
            perror("Read failed");
        }
    }
} else if (select_ret == 0) {
    printf("Timeout: No data received from device\n");
} else {
    perror("Select error");
}

To verify your write was successful, you need to check two key things: the return value of write(), and optionally wait for device acknowledgment.

Step 1: Validate the Write Return Value #

The write() function returns the number of bytes actually written. Compare this to the number of bytes you intended to send (note: don't send the null terminator unless your device expects it—use strlen() instead of sizeof()):

char write_buff[] = "q,e";
int expected_bytes = strlen(write_buff);
int bytes_written = write(fd1, write_buff, expected_bytes);

if (bytes_written == expected_bytes) {
    printf("Successfully wrote all %d bytes: %s\n", bytes_written, write_buff);
} else if (bytes_written == -1) {
    perror("Write failed entirely");
} else {
    printf("Only wrote %d of %d bytes\n", bytes_written, expected_bytes);
}

Step 2: Wait for Data to Be Transmitted #

Use tcdrain() to ensure all data in the output buffer is sent to the device before proceeding:

tcdrain(fd1); // Blocks until all output is transmitted

Step 3: (Optional) Read a Device Acknowledgment #

Many devices send an ACK message after receiving valid data. If your device does this, use the reading logic from question 1 to listen for this ACK as confirmation the data was received and processed.

Here's the complete, fixed version of your main function:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/select.h>

int main() 
{
    int fd1, retval;
    struct termios usb_parm;

    // Open port with O_RDWR (simpler than separate read/write flags)
    fd1 = open("/dev/ttyACM0", O_RDWR | O_NOCTTY);
    if (fd1 > 0) {
        printf("Device detected\n");
    } else {
        perror("Failed to open port");
        return -1;
    }

    // Configure serial port parameters
    tcgetattr(fd1, &usb_parm);

    // Set custom baud rate
    usb_parm.c_cflag &= ~CBAUD;
    usb_parm.c_cflag |= BOTHER;
    usb_parm.c_ispeed = 9600;
    usb_parm.c_ospeed = 9600;

    // Set raw mode
    usb_parm.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
    usb_parm.c_oflag &= ~OPOST;
    usb_parm.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
    usb_parm.c_cflag &= ~(CSIZE | PARENB);
    usb_parm.c_cflag |= CS8;

    // Apply settings
    retval = tcsetattr(fd1, TCSANOW, &usb_parm);
    if (retval != 0) {
        perror("Failed to set serial attributes");
        close(fd1);
        return -1;
    }

    // Flush pending data
    tcflush(fd1, TCIOFLUSH);

    // Write data to device
    char write_buff[] = "q,e";
    int expected_bytes = strlen(write_buff);
    int bytes_written = write(fd1, write_buff, expected_bytes);

    if (bytes_written == expected_bytes) {
        printf("Successfully wrote all %d bytes: %s\n", bytes_written, write_buff);
        tcdrain(fd1); // Wait for transmission to finish
    } else {
        perror("Write failed");
        close(fd1);
        return -1;
    }

    // Read response
    char read_buff[64] = {0};
    int bytes_read = 0;
    fd_set readfds;
    struct timeval tv;

    tv.tv_sec = 2;
    tv.tv_usec = 0;
    FD_ZERO(&readfds);
    FD_SET(fd1, &readfds);

    int select_ret = select(fd1 + 1, &readfds, NULL, NULL, &tv);
    if (select_ret > 0 && FD_ISSET(fd1, &readfds)) {
        int byteavailable = 0;
        ioctl(fd1, FIONREAD, &byteavailable);
        if (byteavailable > 0) {
            bytes_read = read(fd1, read_buff, sizeof(read_buff) - 1);
            if (bytes_read > 0) {
                read_buff[bytes_read] = '\0';
                printf("Read buff=%s\n", read_buff);
                printf("Bytes read=%d\n", bytes_read);
            } else {
                perror("Read failed");
            }
        }
    } else if (select_ret == 0) {
        printf("Timeout: No response from device\n");
    } else {
        perror("Select error");
    }

    close(fd1);
    return 0;
}

内容的提问来源于stack exchange，提问作者user9478215