Mahesh,
I assume that the reason you can't service your watchdog timer often enough to meet 500ms requirement is that the processor is in a timing critical routine and that routine takes 500ms to complete. If this is the case, can you just temporarily disable the watchdog? I mean, if that routine is so timing concern, you may want to disable all interrupts any way.
You can also build your own watchdog circuit. It is nothing more than a capacitor, a diode and a voltage comparator that controls your micro's RESET line. You can adjust the size of the cap to meet your requirement.