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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; }