Hello, I'm trying to write a serial port routine that uses the PEC unit for transmit and receive. Target processor is C167CR-LM. I got this routines working when using the S0TINT interrupt service with PEC transfer, but not when using the S0TBINT. I tracked down the problem to a single byte PEC transfer, where sometimes one expected interrupt is missing. For a single byte PEC transfer I expect two interrupts: - the PEC 1->0 transfer, that leaves S0TBIR set - the 'real' transmitter buffer empty S0TBIR Unfortunately, I often get only 1 of this interrupts. Could this problem arise from the fact that the first byte written to S0TBUF is immediately moved to the transmit register and therefor the interrupts from the PEC and transmitter buffer empty come to the same time and can't be distinguished by the hardware? This could explain why my routines work when using the S0TINT. Here the PEC 1->0 S0TIR is immediately active but the second S0TIR is rising after last bit of the transmitter is sent. Is this a correct interpretation? Is there another way to check that the interrupt is caused by the TBUF-empty and not by the PEC 1->0 change? - Heinz