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“不定長度”。一個(gè)實(shí)例如下圖：銳米LoRa終端的通信協(xié)議幀。
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如果一個(gè)系統(tǒng)接收上述“不定長度”的協(xié)議幀，將會(huì)有一個(gè)挑戰(zhàn)--如何高效接收與解析。

為簡化系統(tǒng)設(shè)計(jì)，我們強(qiáng)烈建議您采用“狀態(tài)機(jī)”來解析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：
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那么ISR處理這個(gè)狀態(tài)機(jī)來得及嗎？答案是：so easy！因?yàn)樗挥?個(gè)動(dòng)作，運(yùn)算量十分?。?br />
比較接收數(shù)據(jù) -> 更新狀態(tài)變量 -> 存儲(chǔ)接收數(shù)據(jù)，C語言僅3條語句，翻譯成機(jī)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; 
}

審核編輯：湯梓紅