hi;
thank you for your replies.the reason i asked this question is as follows:

i am making a counting system for a radiation detector.i also want to include self check routine.i thought about an arrangement that invloves a 2:1 digital mux.mux inputs are from
a. radiation detector
b.onboard frequency generator.
i select one of them as per requirement.asume that d-mux is going to last forever.so if i do not get pulsed from detector for some time i select onboard frequency.if it gives desired count detector is fail.BUT if onchip counter(t0/t1/t2)has failed then i cant really be sure whether the zero pulse is because of counter failure or because of detector failure.
thanks
pranav phatak