The code:

    ...
WORKLOOP: 
    CPL   LEDPIN 
    JMP   WORKLOOP
    ...

Will indeed produce a 50% duty cycle signal on the LEDPIN if that was an output port bit AND there were no interrupts occuring. Any interrrupt from within this tight loop will cause the LEDPIN to have the current high or low period of the output signal extended by what ever time it takes to execute the interrupt routine.

(Now note that if you did the non-standard thing of jumping out of the interrupt routine that was discussed in another thread here over the last week, then the waveform on LEDPIN would stop becasue the code never came back to this tight loop).

Michael Karas