We have seen that the design of mod-2n binary up-counters is straightforward, with all T states produced, and cycled through in sequence. For general mod-N counters we must begin to use (simplified) synchronous sequential circuit design techniques, with each present state of the circuit used to produce the necessary steering logic to take the circuit into the desired next state. Note that because the circuit is docked the outputs are always stable. This is because although the next state is determined by the present slate, the next state and present state variables are separated by the flip-flops, unlike the asynchronous sequential circuits studied in Chapter 5.
We begin by redesigning the mod-8 counter, which should give the circuit produced above (see the end of this section). Firstly we look at how D-type flip- flops could be used and then JK-types. This will also give us our first insight into the general differences in circuits designed using these two types of flip-flop.