Rules for counting down are 1101, 1100... 0000 then again 1101. Or do you mean something else by rules for counting down/up?Asynchronous counters are easy; as long as they count in the normal sequence. Do you know the rules for count down (or up)?
The rules tell you how to connect flip flops to count down based on the polarity of their clocks.Or do you mean something else by rules for counting down/up?