What formula is the simulator using to come up with the prescale values for the Analog Devices ADuC831 WDT? The prescale values listed in the drop down list change depending on the operating frequency selected. The period shouldn't be depend on the operating clock frequency. The specification sheet for the ADuC831 (Rev 0 Page 44) shows the time period should be as follows....
twd = (2^(PreScaler)* 2^9 /(fRC OSC))