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：
/**
* [url=home.php?mod=space&uid=247401]@brief[/url] 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.
* [url=home.php?mod=space&uid=536309]@NOTE[/url] 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;
}
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