Dual Decade Counters

This circuit contains eight master-slave flip-flops and additional gating to implement two individual four-bit counters. The HD74LS390 incorporates dual divide-by-two and divide-by-five counters, which can be used to implement cycle lengths equal to any whole and / or cumulative multiples of 2 and / or 5 up to divide-by-100. When connected as a biquinary counter, the separate divide-by-two circuit can be usedto provide symmetry (a square wave) at the final output stage.