锐米LoRa终端的通信协议帧

通信设计中考虑协议的灵活性，经常把协议设计成“不定长度”。

一个实例如下图：锐米LoRa终端的通信协议帧。

如果一个系统接收上述“不定长度”的协议帧，将会有一个挑战--如何高效接收与解析。

为简化系统设计，我们强烈建议您采用“状态机”来解析UART数据帧，并且把解析工作放在ISR（中断服务程序）完成，仅当接收到最后一个字节（0x0D）时，再将整个数据帧提交给进程处理。

该解析状态机的原理如下图所示：

那么ISR处理这个状态机来得及吗？答案是：so easy！因为它只有3个动作，运算量十分小：

比较接收数据 -> 更新状态变量 -> 存储接收数据，C语言仅3条语句，翻译成机器指令也不超过10条。

代码清单如下：

/*** @brief  Status of received communication frame*/typedef enum{    STATUS_IDLE = (uint8_t)0,    STATUS_HEAD, /* Rx Head=0x3C */    STATUS_TYPE, /* Rx Type */    STATUS_DATA, /* Data filed */    STATUS_TAIL, /* Tail=0x0D */    STATUS_END, /* End of this frame */} COMM_TRM_STATUS_TypeDef;

/*** @brief  Data object for received communication frame*/typedef struct{    uint8_t    byCnt; /* Count of 1 field */    uint8_t    byDataLen; /* Length of data field */    uint8_t    byFrameLen; /* Length of frame */    COMM_TRM_STATUS_TypeDef    eRxStatus;    uint8_t    a_byRxBuf[MAX_LEN_COMM_TRM_DATA];} COMM_TRM_DATA;

/**

* @brief  Data object for received communication frame.* @note  Prevent race condition that accessed by both ISR and process.*/static COMM_TRM_DATA    s_stComm2TrmData;

/**  * @brief  Put a data that received by UART into buffer.  * @note  Prevent race condition this called by ISR.   * @param  uint8_t byData: the data received by UART.  * @retval  None  */void comm2trm_RxUartData(uint8_t byData){    /* Update status according to the received data */    switch (s_stComm2TrmData.eRxStatus)    {        case STATUS_IDLE:            if (COMM_TRM_HEAD == byData) /* Is Head */            {                s_stComm2TrmData.eRxStatus = STATUS_HEAD;            }            else            {                goto rx_exception;            }            break;        case STATUS_HEAD:            if (TYPE_INVALID_MIN < byData && byData < TYPE_INVALID_MAX) /* Valid type */            {                s_stComm2TrmData.eRxStatus = STATUS_TYPE;            }            else            {                goto rx_exception;            }            break;        case STATUS_TYPE:            if (byData <= MAX_LEN_UART_FRAME_DATA) /* Valid data size */            {                s_stComm2TrmData.eRxStatus = STATUS_DATA;                s_stComm2TrmData.byDataLen = byData;            }            else            {                goto rx_exception;            }            break;        case STATUS_DATA:            if (s_stComm2TrmData.byCnt < s_stComm2TrmData.byDataLen)            {                ++s_stComm2TrmData.byCnt;            }            else            {                s_stComm2TrmData.eRxStatus = STATUS_TAIL;            }            break;        case STATUS_TAIL:            if (COMM_TRM_TAIL == byData)            {                /* We received a frame of data, now tell process to deal with it! */                process_poll(&Comm2TrmProcess);            }            else            {                goto rx_exception;            }            break;        default:            ASSERT(!"Error: Bad status of comm2trm_RxUartData().
");            break;    }

    /* Save the received data */    s_stComm2TrmData.a_byRxBuf[s_stComm2TrmData.byFrameLen++] = byData;    return;

rx_exception:    ClearCommFrame();    return;}