Electronic – FT230X Baud Rate Aliasing

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.

I once had a similar problem with a microcontroller which didn't have a UART nor exactly known clock frequency (crystal was too expensive). I let the other party send a specific byte, for instance 0x01 and measured the time the bus was low. This was 8 bit times (startbit + 7 zeros of data). So dividing this by 8 I got the value I should set my timer to.
So, before you configure the I/O pins for UART use them as common I/O until the Bluetooth module sent this agreed upon character and time it. You could do this running a loop, or use timer capture to detect the falling and rising edges (more accurate).