c language-embedded album 2~

A learning summary post for myself~~ whaosoft aiot http://143ai.com Here is only the second season of embedded related codes in c language

1. Efficiently parse protocol frames of variable length

   The flexibility of the protocol is considered in the communication design, and the protocol is often designed as "indefinite length". An example is shown in the figure below: the communication protocol frame of RMI LoRa terminal.  If a system receives the above-mentioned "indefinite length" protocol frame, there will be a challenge - how to efficiently receive and parse it.

    In order to simplify the system design, we strongly recommend that you use a "state machine" to parse the UART data frame, and put the parsing work in the ISR (Interrupt Service Routine) to complete, and only when the last byte (0x0D) is received, the entire The data frame is submitted to the process for processing. The principle of the parsing state machine is shown in the figure below:  So is it too late for the ISR to process this state machine? The answer is: so easy! Because it only has 3 actions, the amount of calculation is very small:

    Compare received data -> update state variables -> store received data, there are only 3 statements in C language, and no more than 10 machine instructions when translated.

    The code listing is as follows:

/**
* @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; 
}