30 CAN回环模式下示波器测CAN_TX无法收到波形？

发布于 2022-03-10 11:22:29 浏览：2048 订阅该版

如题所示，下图是我在RT-Thread Studio串口终端打印到的数据，开启了can_rx_thread接收来自main()主函数线程发送的数据
![1.jpg](https://oss-club.rt-thread.org/uploads/20220310/56e06a903fd5dfea73a2c10f6715a962.jpg.webp)

下面是main主线程发送数据代码，发送和接收的代码都是用例程给的，发送模式设置成环回模式，我的理解是STM32内置的bxCAN控制器只能向总线发送数据，接收不到总线数据，并且CAN_TX引脚在控制器内部就将数据交给CAN_RX

我在无RT_Thread情况下，回环模式和正常模式都调通了，并且这两种模式测得都有波形，并且PC中CAN分析仪的上位机软件可以收到CAN帧
```
/*CAN设备操作*/
    rt_err_t can_ret = 0;
    rt_size_t size = 0;
    struct rt_can_msg msg = { 0 };
    //can1使用PA11 PA12引脚
    can_dev = rt_device_find(CAN_DEV_NAME);
    if (can_dev == RT_NULL)
    {
        LOG_E("rt_device_find [can1] failed...\n");
        return -EINVAL;
    }
    //以中断接收及发送模式打开 CAN设备
    can_ret = rt_device_open(can_dev, RT_DEVICE_FLAG_INT_TX | RT_DEVICE_FLAG_INT_RX);
    if (can_ret < 0)
    {
        LOG_E("rt_device_open [can1] failed...\n");
        return can_ret;
    }
    //设置 CAN 通信的波特率为 500kbit/s
    can_ret = rt_device_control(can_dev, RT_CAN_CMD_SET_BAUD, (void *) CAN500kBaud);
    can_ret = rt_device_control(can_dev, RT_CAN_CMD_SET_MODE, (void *)RT_CAN_MODE_LOOPBACK);
    //获取 CAN 总线设备的状态
    can_ret = rt_device_control(can_dev, RT_CAN_CMD_GET_STATUS, &status);
    LOG_D("can bus current status:%d", can_ret);
    //初始化CAN接收信号量
    rt_sem_init(&can_sem, "can_sem", 0, RT_IPC_FLAG_FIFO);
    //创建CAN接收线程，并开启
    can_th = rt_thread_create("can_rx", can_rx_thread, RT_NULL, 1024, 25, 10);
    if (can_th != RT_NULL)
    {
        rt_thread_startup(can_th);
    }
    else
    {
        rt_kprintf("create can_rx thread failed!\n");
    }
    while (1)
    {
        msg.id = 0x78; /* ID 为 0x78 */
        msg.ide = RT_CAN_STDID; /* 标准格式 */
        msg.rtr = RT_CAN_DTR; /* 数据帧 */
        msg.len = 8; /* 数据长度为 8Byte */
        /* 待发送的 8 字节数据 */
        msg.data[0] = 0x30;
        msg.data[1] = 0x31;
        msg.data[2] = 0x32;
        msg.data[3] = 0x33;
        msg.data[4] = 0x34;
        msg.data[5] = 0x35;
        msg.data[6] = 0x36;
        msg.data[7] = 0x37;
        /* 发送一帧 CAN 数据 */
        size = rt_device_write(can_dev, 0, &msg, sizeof(msg)); //&msg：CAN帧起始地址
        if (size == 0)
        {
            rt_kprintf("can_dev write data failed!\n");
        }
        rt_thread_delay(1000);
    }
```

以下是can_rx_thread接收数据代码
```
static void can_rx_thread(void *parameter)
{
    int i;
    rt_err_t res;
    struct rt_can_msg rxmsg = { 0 };

/* 设置接收回调函数 */
    rt_device_set_rx_indicate(can_dev, can_rx_callback);

#ifdef RT_CAN_USING_HDR
    struct rt_can_filter_item items[5] = { RT_CAN_FILTER_ITEM_INIT(0x100, 0, 0, 0, 0x700, RT_NULL, RT_NULL), /* std,match ID:0x100~0x1ff，hdr 为 - 1，设置默认过滤表 */
    RT_CAN_FILTER_ITEM_INIT(0x300, 0, 0, 0, 0x700, RT_NULL, RT_NULL), /* std,match ID:0x300~0x3ff，hdr 为 - 1 */
    RT_CAN_FILTER_ITEM_INIT(0x211, 0, 0, 0, 0x7ff, RT_NULL, RT_NULL), /* std,match ID:0x211，hdr 为 - 1 */
    RT_CAN_FILTER_STD_INIT(0x486, RT_NULL, RT_NULL), /* std,match ID:0x486，hdr 为 - 1 */
    { 0x555, 0, 0, 0, 0x7ff, 7, } /* std,match ID:0x555，hdr 为 7，指定设置 7 号过滤表 */
    };
    struct rt_can_filter_config cfg = { 5, 1, items }; /* 一共有 5 个过滤表 */
    /* 设置硬件过滤表 */
    res = rt_device_control(can_dev, RT_CAN_CMD_SET_FILTER, &cfg);
    RT_ASSERT(res == RT_EOK);
#endif

while (1)
    {
        /* hdr 值为 - 1，表示直接从 uselist 链表读取数据 */
        rxmsg.hdr = -1;
        /* 阻塞等待接收信号量 */
        rt_sem_take(&can_sem, RT_WAITING_FOREVER);
        /* 从 CAN 读取一帧数据 */
        rt_device_read(can_dev, 0, &rxmsg, sizeof(rxmsg));
        /* 打印数据 ID 及内容 */
        rt_kprintf("ID:%x", rxmsg.id);
        for (i = 0; i < 8; i++)
        {
            rt_kprintf("%2x", rxmsg.data[i]);
        }

rt_kprintf("\n");
    }
}
```

下图是示波器测不到波形，探头接CAN_TX，鳄鱼夹接GND
![2.jpg](https://oss-club.rt-thread.org/uploads/20220310/a62973b06e680aef659b10de9e07876b.jpg.webp)

使用正点原子STM32F103RBT6 开发板，无外部高速晶振HSE，所以PLL倍频后时钟SYSCLK最高速=64MHz，经过AHB，APB1预分频后APB1CLK=32MHz（在drv_clk.c的system_clock_config()配置），导致我修改了drv_can.c中的波特率数组 CAN500kBaud, (CAN_SJW_2TQ | CAN_BS1_4TQ  | CAN_BS2_3TQ | 8)}，即（32/(3+4+1)*8=500KHz），修改的原因是原数据是使用APB1CLK=36MHz来计算的，见下图（看英文的绿色注释）
![4.jpg](https://oss-club.rt-thread.org/uploads/20220310/395e6ff13123ae1e3e301be08743fb4c.jpg.webp)

并且我在debug模式下跑过整个流程，各个寄存器的值在执行过程中的变化是没问题的，按道理来说是不会出现测不到波形的状况，求赐教

下图是debug模式中设置完500kHz波特率后bxCAN控制器中寄存器的值
![3.jpg](https://oss-club.rt-thread.org/uploads/20220310/d63e5ff9ced8ec2d0d13848cbcab0066.jpg.webp)