UART message creation

This is a description of how to create a UART message for the driver and common block API.

UART message structure

Byte	Description
0	session | 0x80
1	(session | 0x80)^0x55
2	Message lenght -1
3	SID
4	Receiver address
5	Payload bit 0
6	Payload bit 1
7	Payload bit 2
5 + n	Payload bit n
Last byte	CRC

How to calculate CRC

It is XOR from bytes in the UART message. The First 2 bytes are not counted to CRC.

crc = Byte2 ^ Byte3 ^ Byte4 ^ ... ^ Last_byte

Example of code to create the UART frame in C

/*!
    \brief Create the UART frame

    \param[in] data_len - Number of bytes to send
    \param[in] data - data to send
                0 byte of data = SID
                1 byte of data = Receiver addr
                2-end bytes = Payload
    \param[in] txbuf - Output buffer
    \param[in] tx_len - Number of bytes in output buffer

    \return int32_t
            0 - succes
            -1 - output (tx) buffer is small
*/
int32_t
umsg_send_msg(uint8_t data_len, uint8_t* data, uint8_t* txbuf, uint8_t tx_len)
{
    static uint8_t session = 0;

    if (data_len + 4 > tx_len)
        return -1;

    uint8_t* ptr = txbuf;
    uint8_t i;
    session |= 0x80;
    *ptr++ = session;
    *ptr++ = session ^ 0x55;
    *ptr++ = data_len - 1;

    uint8_t crc = 0;
    crc ^= data_len - 1;

    for (i = 0; i < data_len; i++) {
        crc ^= *data;
        *ptr++ = *data++;
    }

    *ptr++ = crc;
    session++;

    return 0;
}