RT-Thread-用DMA发送的方式，串口无法发出数据，为什么？RT-Thread问答社区

5 用DMA发送的方式，串口无法发出数据，为什么？

发布于 2023-08-11 09:14:39 浏览：579 订阅该版

看老代码发现的bug。修改波特率的时候，先关闭串口设备，然后再打开，打开的时候多加了一个RT_DEVICE_FLAG_DMA_TX，此时，`rt_device_write(env_dev, 0, string, strlen(string));`这样最基础的发送函数都发不出来。
初始化时代码如下

```c
struct serial_configure pconfig;
	/* 初始化串口 */
    if (env_dev == RT_NULL)
    {
        env_dev = rt_device_find(ENV_NAME);
        RT_ASSERT(env_dev);

pconfig.baud_rate = BAUD_RATE_9600;
        pconfig.data_bits = DATA_BITS_8;
        pconfig.stop_bits = STOP_BITS_1;
        pconfig.parity    = PARITY_NONE;
        pconfig.bit_order = BIT_ORDER_LSB;
        pconfig.invert    = NRZ_NORMAL;
        pconfig.bufsz     = 4096;
        pconfig.reserved  = 0;
        /* 设置串口参数 */
        rt_device_control(env_dev, RT_DEVICE_CTRL_CONFIG, &pconfig);
        /*设置串口的回调函数 */
        rt_device_set_rx_indicate(env_dev, env_recv_ind);
    }
//	if(RT_EOK ==rt_device_open(env_dev, RT_DEVICE_FLAG_INT_RX | RT_DEVICE_FLAG_DMA_TX))
	if(RT_EOK ==rt_device_open(env_dev, RT_DEVICE_FLAG_INT_RX ))
	{
		LOG_D("Screen serial open success.\r\n");
//            return RT_EOK;
	}
	else
	{
		LOG_D("Screen serial open failure.\r\n");
//		return RT_ERROR;
	}

```

修改波特率时的代码如下

```c
tmp_baud = (tmp_baud == 1)?2:1;
												struct serial_configure pconfig;
												if (tmp_baud == 1)
												{
													pconfig.baud_rate = BAUD_RATE_9600;
												}
												else if (tmp_baud == 2)
												{
													pconfig.baud_rate = BAUD_RATE_115200;
												}
												sys_para.env_bdrate = tmp_baud;//修改全局变量
//												USART3->BRR = 
												pconfig.data_bits = DATA_BITS_8;
												pconfig.stop_bits = STOP_BITS_1;
												pconfig.parity    = PARITY_NONE;
												pconfig.bit_order = BIT_ORDER_LSB;
												pconfig.invert    = NRZ_NORMAL;
												pconfig.bufsz     = 4096;
												pconfig.reserved  = 0;
												rt_device_close(env_dev);
												/* 设置串口参数 */
												rt_device_control(env_dev, RT_DEVICE_CTRL_CONFIG, &pconfig);
//												rt_device_open(env_dev, RT_DEVICE_FLAG_INT_RX | RT_DEVICE_FLAG_DMA_TX);
												rt_device_open(env_dev, RT_DEVICE_FLAG_INT_RX );

```

打开的时候`rt_device_open(env_dev, RT_DEVICE_FLAG_INT_RX | RT_DEVICE_FLAG_DMA_TX);` 此时旧无法发送。

很奇怪，env_dev是串口3，也没有和别的DMA有冲突啊，就算有冲突也可以仲裁啊。谢谢

补充一下发送的内容

```c
//数组是全局变量
static const rt_uint8_t cmd1[] = {0x01, 0x03, 0x00, 0x02, 0x00, 0x02, 0x65, 0xCB};//light
static const rt_uint8_t cmd2[] = {0x02, 0x03, 0x00, 0x02, 0x00, 0x01, 0x25, 0xF9};//ch5
static const rt_uint8_t cmd3[] = {0x03, 0x03, 0x00, 0x02, 0x00, 0x01, 0x24, 0x28};//h2s
static const rt_uint8_t cmd4[] = {0x04, 0x03, 0x00, 0x00, 0x00, 0x02, 0xC4, 0x5E};//pm2.5

//发送部分代码
//根据索引
            if (index == 0)
            {
                rt_kprintf("write cmd1\n");
                rt_device_write(serial, 0, cmd1, 8);
            }
            else if (index == 1)
            {
                rt_kprintf("write cmd2\n");
                rt_device_write(serial, 0, cmd2, 8);
            }
            else if (index == 2)
            {
                rt_kprintf("write cmd3\n");
                rt_device_write(serial, 0, cmd3, 8);
            }
            else if (index == 3)
            {
                rt_kprintf("write cmd4\n");
                rt_device_write(serial, 0, cmd4, 8);
            }
            index++;
            if (index == 4)
            {
                index = 0;
            }

```