通信設計中考慮協議的靈活性，經常把協議設計成“不定長度”。

一個實例如下圖：銳米LoRa終端的通信協議幀。

如果一個系統接收上述“不定長度”的協議幀，將會有一個挑戰--如何高效接收與解析。

為簡化系統設計，我們強烈建議您采用“狀態機”來解析UART數據幀，并且把解析工作放在ISR（中斷服務程序）完成，僅當接收到最后一個字節（0x0D）時，再將整個數據幀提交給進程處理。

該解析狀態機的原理如下圖所示：

那么ISR處理這個狀態機來得及嗎？答案是：so easy！因為它只有3個動作，運算量十分小：

比較接收數據 -> 更新狀態變量 -> 存儲接收數據，C語言僅3條語句，翻譯成機器指令也不超過10條。

代碼清單如下：

/*** @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;}