How to set a byte of start of message for a UART transmission

Question

I'm trying to transmit with UART, from an stm32 Nucleo to Matlab Simulink, a certain amount of data. More precisely I want to transmit two uint16 variables. I splitted them in two packeges of 1 byte each. The problem is that when I receive them on Simulink, it's not garanteed that SImulink take the packages in the correct order, so I have to implement something that allows me to understand where is the start of the message. I'm trying to do that, but at the moment I'm not succeeding. I want to specify that in Simulink I can write the header of the message, so I think that I just have to set it in my stm32 board.

Here it is my code:

//Serial Stream Routine
         if ((streamActive != 0) && (dataReady == 1))
         {
             do{
                     //Sending the first byte of counter
                     firstbyte = counter & 0xFF;
                     memcpy(str, &firstbyte, 1);
                     HAL_UART_Transmit(&huart2, str, 1, HAL_MAX_DELAY);
                     //Sending the second byte of counter
                     secondbyte =(counter >> 8) & 0xFF ;
                     memcpy(str, &secondbyte, 1);
                     HAL_UART_Transmit(&huart2, str, 1, HAL_MAX_DELAY);

                     if(streamActive == 2){ //We transmit the counter with an offset

                         offset = counter + OFFSET_VALUE;

                         //Sending the first byte of offset
                         firstbyte = offset & 0xFF;
                         memcpy(str, &firstbyte, 1);
                         HAL_UART_Transmit(&huart2, str, 1, HAL_MAX_DELAY);
                         //Sending the second byte of offset
                         secondbyte =(offset >> 8) & 0xFF ;
                         memcpy(str, &secondbyte, 1);
                         HAL_UART_Transmit(&huart2, str, 1, HAL_MAX_DELAY);
                     }

                     counter++;
                     dataReady = 0;
                 }while(counter < MAX_VALUE);

So I want to trasmit the variables counter and offset. Help me, thanks.

Answer 1

Now I want to receive them in the correct order

Add header & PC & ARM Corex are both little-endian so it is enough to:

         do{
                 HAL_UART_Transmit(&huart2, (void *)"START", 5, HAL_MAX_DELAY);
                 HAL_UART_Transmit(&huart2, (void *)&counter, 2, HAL_MAX_DELAY);
                 if(streamActive == 2){ //We transmit the counter with an offset
                     offset = counter + OFFSET_VALUE;
                     HAL_UART_Transmit(&huart2, (void *)"START", 5, HAL_MAX_DELAY);
                     HAL_UART_Transmit(&huart2, (void *)&offset, 2, HAL_MAX_DELAY);
                 }
                 counter++;
                 dataReady = 0;
            }while(counter < MAX_VALUE);

And in simulink (matlab command): uint16('START')