Editing Counter (digital) (section)

===Synchronous vs. asynchronous (ripple) <span class="anchor" id="Ripple counter"></span>===

Counters are broadly categorized as either synchronous or asynchronous.

In synchronous counters, all flip-flops share a common clock and change state at the same time.<ref name="Keslin"/>

In an asynchronous counter, also known as a '''ripple counter''',<ref name="Maini" /> each flip-flop has a unique clock, and the flip-flop states change at different times. Each flip-flop introduces a delay from input clock edge to output toggle. This causes the counter bits to change at different times, thus producing a ripple effect and making the count unstable as the counter input clock propagates through the circuit. The duration of this instability (the output settling time) is proportional to the number of flip-flops. This makes ripple counters unsuitable for use in [[synchronous circuit]]s that require the counter to have a fast output settling time.<ref name="Gorla">{{cite web |last1=Gorla |first1=Raju |title=Counters - Digital Circuits |url=https://vlsiweb.com/counters/ |publisher=VLSI Web |access-date=9 May 2025}}</ref> Also, it is often impractical to use ripple counter output bits as clocks for external circuits because the ripple effect causes timing skew between the bits. Ripple counters are commonly used as general-purpose counters and clock frequency dividers in applications where the instantaneous count and timing skew is unimportant. Asynchronous counters are typically not used in VLSI ICs due to the difficulties of simulating and testing them and because they require much greater design effort to ensure reliable operation.<ref name="Keslin"/>