Electronic – Tiva-C Launchpad – Sending a Tx signal over long distance

How do you get the characters from the serial port buffer register? If you simply read them in a loop like

void main() {       
  char c;
  while (1) {
    c=SBUF;
    do_something(c);
  }
}

Then you will miss characters once the execution time of do_something() gets longer. Note that this includes time spend in intterrupts. The 8051 serial port has no hardware fifos, a character will be overwritten by the next one if it was not read in time.

Our solution was to read the characters into a Ringbuffer during the serial Intterupt, and to use the FIFO in the main loop. Works (with high priority interrupt) for 460800 Baud with 7,3728 MHz crystal on a Silabs 80C51FXXX.

Update

As we now see the source coude, the bug is now clear: You wait in the main loop for your character to be sent. But that means you wait a whole "character time" without being able to read your interrupt character buffer, and another few cyles to detect and read the next. This is too long if the sender sends characters fast, as a PC does.

The higher baudrate makes the UART module transmit faster, so you have more "spacing" between the bytes, but it seems to me that the rate at which you're transmitting the bytes is still the same. Your timing diagrams are missing total time so its hard to see if this is the case. Don't compare last edge to first edge. Compare the first edges of the the two bytes. Are these still the same?