UART Synchronization Algorithm for receiving Stream of Data

Question

I am receiving this frame $0,S1,B2,Kffffffffffff,T61*34 through UART with this code.

//Receive Data
for(uint8_t i = 0; i < size; i++){
    receivedFrame[i] = EUSART1_Read();
    if(receivedFrame[i] == '*'){
        size = i + 3;
    }
}

The start of the frame is always $ and the end is always * after that comes two bytes holds the check sum for the previous bytes (ex 34).

The frame length is not fixed but it has a minimum length of 26(from $ to *) + 2 bytes of check sum and maximum length of 62 and also + 2 bytes of check sum.

but it is not the best thing to use since so many scenarios could happen to make this unstable for example if the * didn't arrive, this will make every frame I read is wrong.

I searched for better way to receive data through UART but didn't manage to find any thing. I am looking for better ways to deal with receiving frames like this.

Answer 1

This solution is based on the assumption that there is no trailing CR or LF, but it will also work in this case by skipping CR and LF until $ is found.

I suggest to combine your algorithm with a check for $ which whould reset the position to 0 and store the $ at position 0. This will ignore frames that don't contain * at the expected position.

You probably should initialize size to the maximum before entering the loop and make sure that size is not increased in case the * is received too late.

Additionally I would make sure that the first character is '$' and not increment the index if this is not true.

For this conditional incrementing of the index, a while loop is better suited.

Untested proposal:

unsigned char receivedFrame[64];
uint8_t size = sizeof(receivedFrame);
uint8_t i = 0;
while(i < size)
{
    unsigned char ch = EUSART1_Read();
    if(ch == '$')
    {
        // in case i was > 0, ignore wrong frame, start from beginning
        i = 0;
    }
    else if(receivedFrame[i] == '*'){
        uint8_t new_size = i + 3;
        if(new_size < size)
        {
            size = new_size;
        }
    }
    receivedFrame[i] = ch;
    // stay at the first position if the character is not '$'
    if((i > 0) || (ch == '$'))
    {
        i++;
    }
}

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Additional hint: For testing the algorithm on a PC you could replace EUSART1_Read with fgetc or getchar. Note that these functions return an int value which should be checked for EOF before converting it to an unsigned char.