通信設(shè)計(jì)中考慮協(xié)議的靈活性，經(jīng)常把協(xié)議設(shè)計(jì)成“不定長(zhǎng)度”。

一個(gè)實(shí)例如下圖：銳米LoRa終端的通信協(xié)議幀。

如果一個(gè)系統(tǒng)接收上述“不定長(zhǎng)度”的協(xié)議幀，將會(huì)有一個(gè)挑戰(zhàn)--如何高效接收與解析。

為簡(jiǎn)化系統(tǒng)設(shè)計(jì)，我們強(qiáng)烈建議您采用“狀態(tài)機(jī)”來(lái)解析UART數(shù)據(jù)幀，并且把解析工作放在ISR（中斷服務(wù)程序）完成，僅當(dāng)接收到最后一個(gè)字節(jié)（0x0D）時(shí)，再將整個(gè)數(shù)據(jù)幀提交給進(jìn)程處理。

該解析狀態(tài)機(jī)的原理如下圖所示：

那么ISR處理這個(gè)狀態(tài)機(jī)來(lái)得及嗎？答案是：so easy！因?yàn)樗挥?個(gè)動(dòng)作，運(yùn)算量十分?。?/span>

比較接收數(shù)據(jù) -> 更新?tīng)顟B(tài)變量 -> 存儲(chǔ)接收數(shù)據(jù)，C語(yǔ)言僅3條語(yǔ)句，翻譯成機(jī)器指令也不超過(guò)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;}