I have made and programmed a demultiplexer for the DMX512 protocol which decodes the received signal and outputs the individual signals onto multiple output signals in order to trigger a circuit which has lights connected to it. It uses the P89C51(RC+) MCU.

While I programmed this, it was running on an 8 MHz quartz, which worked really fine. The only problem with that was that there wasn't enough time to take an address (0 up to 511) into account which was set using dipswitches, without skipping values.

Therefore I've tried to replace it with a 32 MHz quartz (the MCU supports up to 33 MHz). I set T2CON to #034H and SCON to #11001000B. RCAP2H is set to #0FFH and RCAP2L is temporarily made configurable using the dipswitches. According to Keil's baudrate calculator the RCAP2L value should be #0FCH (as opposed to #0FFH if running at 8 MHz). So that's the value that I tried and I've tried many values that are near it.

Still if trying it, the results seem completely unpredictable even though if I replace the original quartz again and set the dipswitches to #0FF it is working fine (except for skipping of values).

Does anyone know how this problem could be caused and a possible resolution for it? I've done many searches on the internet (and of course at this site as well) but unfortunately no avail. All those internet sites seem to be using either 8 or 16 crystals (and using serial port mode 2), even though 32 seems to be supposed to work too. Many thanks for any help.