STM32 UART 是最基础的通信接口。本文介绍一种基于 STM32 UART 的通信协议解析框架。与其说是一种解析框架，不如说是一种解析架构，一种解析逻辑更为准确。

测试环境：

• Master MCU: STM32F103RCT6
• Slave Module：尚鑫航 SXH485 H200 串口摄像机模组
• RTOS：无，裸机系统
• Libraries：STSW-STM32054 3.6.0 标准库

整个工程源码可参考 GitHub 仓库 UART_Parse。

(https://github.com/Alson-tang/UART_Parse)

通信协议

简单介绍下 STM32 与 SXH485 H200 串口摄像机模组 之间的通信协议。

整个帧分为以下 6 个部分：

1. 帧头 (2B)
2. 地址 (1B)
3. 命令 (1B)
4. 数据长度 (2B)
5. 数据 (0B ~ 65535B)
6. CRC 校验 (2B)

本文档 重点在于串口解析框架，故这里以三种命令来介绍解析框架：

1. 测试命令
2. 拍照命令
3. 分包取图命令

命令示例

测试命令

测试命令用于测试通信情况。

示例：

MCU 发：90 EB 01 01 02 00 55 AA C1 C2

SXH485 H200 返回：90 EB 01 01 03 00 00 AA 55 F6 EB

测试命令

拍照命令用于触发 SXH485 H200 拍照。

示例：

MCU 发：90 EB 01 40 04 00 00 02 05 01 C1 C2

SXH485 H200 返回：90 EB 01 40 0B 00 00 BD C4 00 00 63 00 00 02 05 01 E0 EC

分包取图命令

分包取图命令用于获取图片数据。

示例：

MCU 发：90 EB 01 48 06 00 00 00 00 00 00 04 C1 C2

SXH485 H200 返回：90 EB 01 49 00 04 FF D8 … 6B 11

解析逻辑

整个解析逻辑可以概括为：UART 中断 + 定时器 + 状态机

通过 UART 的中断标志位 RXNE 完成数据的接收；通过 UART 的中断标志位 IDLE 来判断帧结束；通过定时器来判断接收超时；通过状态机来解析接收到的帧。

UART 中断接收

在 UART 中断处理函数主要完成以下 2 件事情：

• RXNE 标志位：将从串口接收到的数据暂存到 buf 中
• IDLE 标志位：帧接收完成，关闭 RXNE 和 IDLE 中断，关闭定时器，之后进行帧处理

尽量不要在 RXNE 标志位中去进行帧处理，RXNE 标志位只负责接收数据到 buf 中，中断处理尽量做到简洁。在 IDLE 标志位进行帧处理是因为已提前关闭了 RXNE 和 IDLE 中断，所以中断不会再次触发，可以进行帧处理。

/**
  * @brief  This function handles USARTy global interrupt request.
  * @param  None
  * @retval None
  */
void USART2_IRQHandler(void)
{
    if (USART_GetITStatus(SXH485_H200_UART, USART_IT_RXNE) != RESET) {
        USART_ClearITPendingBit(SXH485_H200_UART, USART_IT_RXNE);

        /* Read one byte from the receive data register */
        g_arr_rx_buf[g_u16_rx_buf_thread_size++] = USART_ReceiveData(SXH485_H200_UART);

        if (g_u16_rx_buf_thread_size == SXH485_H200_UART_RX_BUFF_SIZE) {
            /* Disable the USARTy Receive interrupt */
            USART_ITConfig(SXH485_H200_UART, USART_IT_RXNE, DISABLE);
        }
    } else if (USART_GetITStatus(SXH485_H200_UART, USART_IT_IDLE) != RESET) {
        /* Clear IDLE bit */
        USART_ReceiveData(SXH485_H200_UART);
        USART_ClearITPendingBit(SXH485_H200_UART, USART_IT_IDLE);

        USART_ITConfig(SXH485_H200_UART, USART_IT_RXNE, DISABLE);
        USART_ITConfig(SXH485_H200_UART, USART_IT_IDLE, DISABLE);

        sxh485_h200_timer_stop();
        sxh485_h200_frame_handle();
    }
}

状态机 （帧处理）

整个帧处理基于 状态机 来完成。一个一个字节进行处理，当一个状态完成时才切换到下一个状态。当整个解析完成时，会设置串口接收标志。

typedef enum
{
    SXH485_H200_FRAME_HEADER_1_STATUS,
    SXH485_H200_FRAME_HEADER_2_STATUS,
    SXH485_H200_FRAME_ADDR_STATUS,
    SXH485_H200_FRAME_CMD_STATUS,
    SXH485_H200_FRAME_LEN_L_STATUS,
    SXH485_H200_FRAME_LEN_H_STATUS,
    SXH485_H200_FRAME_DATA_STATUS,
    SXH485_H200_FRAME_CRC_L_STATUS,
    SXH485_H200_FRAME_CRC_H_STATUS,
    SXH485_H200_FRAME_MAX_STATUS,
} sxh485_h200_frame_status;

/**
  * @brief  Command handle.
  * @retval None.
  * @retval None.
  */
void sxh485_h200_frame_handle(void)
{
    uint16_t len = 0;
    uint16_t recv_crc = 0;
    uint16_t calc_crc = 0;

    while (1) {
        switch (s_st_sxh485_h200_frame_status) {
            case SXH485_H200_FRAME_HEADER_1_STATUS: {
                if (g_arr_rx_buf[SXH485_H200_FRAME_HEADER_1_INDEX] != SXH485_H200_FRAME_HEADER_1) {
                    s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_HEADER_1_STATUS;
                    sxh485_h200_recv_flag_set(SXH485_H200_RECV_ERROR);

                    return;
                }

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_HEADER_2_STATUS;

                break;
            }

            case SXH485_H200_FRAME_HEADER_2_STATUS: {
                if (g_arr_rx_buf[SXH485_H200_FRAME_HEADER_2_INDEX] != SXH485_H200_FRAME_HEADER_2) {
                    s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_HEADER_1_STATUS;
                    sxh485_h200_recv_flag_set(SXH485_H200_RECV_ERROR);

                    return;
                }

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_ADDR_STATUS;

                break;
            }

            case SXH485_H200_FRAME_ADDR_STATUS: {
                if (g_arr_rx_buf[SXH485_H200_FRAME_ADDR_INDEX] != s_u8_addr) {
                    s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_HEADER_1_STATUS;
                    sxh485_h200_recv_flag_set(SXH485_H200_RECV_ERROR);

                    return;
                }

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_CMD_STATUS;

                break;
            }

            case SXH485_H200_FRAME_CMD_STATUS: {
                if (g_arr_rx_buf[SXH485_H200_FRAME_CMD_INDEX] != s_u8_cmd) {
                    if ((g_arr_rx_buf[SXH485_H200_FRAME_CMD_INDEX] == SXH485_H200_FRAME_CMD_PIC_GET_RECV) &&
                        (SXH485_H200_FRAME_CMD_PIC_GET == s_u8_cmd)) {
                        s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_LEN_L_STATUS;
                    } else {
                        s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_HEADER_1_STATUS;
                        sxh485_h200_recv_flag_set(SXH485_H200_RECV_ERROR);

                        return;
                    }

                } else {
                    s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_LEN_L_STATUS;
                }

                break;
            }

            case SXH485_H200_FRAME_LEN_L_STATUS: {
                len = 0;
                len = g_arr_rx_buf[SXH485_H200_FRAME_LEN_L_INDEX];

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_LEN_H_STATUS;

                break;
            }

            case SXH485_H200_FRAME_LEN_H_STATUS: {
                uint16_t len_h = g_arr_rx_buf[SXH485_H200_FRAME_LEN_H_INDEX];

                len |= (len_h << 8);

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_DATA_STATUS;

                break;
            }

            case SXH485_H200_FRAME_DATA_STATUS: {
                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_CRC_L_STATUS;

                break;
            }

            case SXH485_H200_FRAME_CRC_L_STATUS: {
                recv_crc = 0;
                recv_crc = g_arr_rx_buf[SXH485_H200_FRAME_DATA_INDEX + len];

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_CRC_H_STATUS;

                break;
            }

            case SXH485_H200_FRAME_CRC_H_STATUS: {
                uint16_t recv_crc_h = g_arr_rx_buf[SXH485_H200_FRAME_DATA_INDEX + len + 1];

                recv_crc |= (recv_crc_h << 8);

                calc_crc = crc16(&g_arr_rx_buf[SXH485_H200_FRAME_ADDR_INDEX], len + 4);
                if (recv_crc != calc_crc) {
                    sxh485_h200_recv_flag_set(SXH485_H200_RECV_CRC_ERROR);
                } else {
                    sxh485_h200_cmd_handle(s_u8_cmd, &g_arr_rx_buf[SXH485_H200_FRAME_DATA_INDEX]);
                    sxh485_h200_recv_flag_set(SXH485_H200_RECV_OK);
                }

                s_st_sxh485_h200_frame_status = SXH485_H200_FRAME_HEADER_1_STATUS;

                return;
            }

            default:
                break;
        }
    }
}

定时器 （超时处理）

MCU 每发送一帧数据给 SXH485 H200 后，启动定时器。在定时器中断处理中完成以下事情：

1. 关闭 RXNE 和 IDLE 中断
2. 停止定时器
3. 将接收标志设置为超时

/**
  * @brief  This function handles TIMx global interrupt request.
  * @param  None
  * @retval None
  */
void TIM6_IRQHandler(void)
{
    if (TIM_GetITStatus(TIM6, TIM_IT_Update) != RESET) {
        TIM_ClearITPendingBit(TIM6 , TIM_FLAG_Update);

        USART_ITConfig(SXH485_H200_UART, USART_IT_RXNE, DISABLE);
        USART_ITConfig(SXH485_H200_UART, USART_IT_IDLE, DISABLE);

        USART_ClearITPendingBit(SXH485_H200_UART, USART_IT_RXNE);
        USART_ReceiveData(SXH485_H200_UART);
        USART_ClearITPendingBit(SXH485_H200_UART, USART_IT_IDLE);

        sxh485_h200_timer_stop();
        sxh485_h200_recv_flag_set(SXH485_H200_RECV_TIMEOUT);
    }
}

应用

以测试命令为例，当 MCU 发送测试命令之后，将串口接收标志设置为 receiving 状态, 一直等待串口接收标志变为非 receiving 状态后，说明帧处理完毕或者超时，返回结果。

/**
  * @brief  Check SXH485 H200 status
  * @param  status: Pointer to SXH485 H200 status
  * @param  timeout: Timeout. Unit in seconds.
  * @retval SXH485 H200 operation status.
  *   This parameter can be a value of @ref sxh485_h200_ops_status.
  */
sxh485_h200_ops_status sxh485_h200_check(sxh485_h200_status* status, uint32_t timeout)
{
    uint8_t send_size = 0;
    sxh485_h200_recv_status recv_status = SXH485_H200_RECEIVING;

    s_u8_cmd = SXH485_H200_FRAME_CMD_TEST;

    sxh485_h200_buff_init();
    send_size = sxh485_h200_test_frame_init(s_u8_addr, g_arr_tx_buf);

    sxh485_h200_send(g_arr_tx_buf, send_size);

    sxh485_h200_recv_flag_set(SXH485_H200_RECEIVING);

    sxh485_h200_timer_cfg(timeout);

    recv_status = sxh485_h200_recv_flag_get();
    while (recv_status == SXH485_H200_RECEIVING) {
        recv_status = sxh485_h200_recv_flag_get();
    }

    if (recv_status == SXH485_H200_RECV_OK) {
        *status = g_st_sxh485_h200_status;
        return SXH485_H200_OK;
    } else {
        return SXH485_H200_FAIL;
    }
}