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().\r\n");break;}/* Save the received data */s_stComm2TrmData.a_byRxBuf[s_stComm2TrmData.byFrameLen++] = byData;return;rx_exception:ClearCommFrame();return;
}
