Suppose that we have a synchronous 4bit counter A that counts from 0 to 9 synchronized with a clock signal CP. We put an AND gate such that the signal Y = Q3*Q0 is 1 when we count a decade. The signal Y is now driven to both another counter B that counts decades and the RESET pin of counter A.

My question is the following:

Which option is better, namely a synchronous or an asynchronous counter, to use for counter B in order to count these periodic phenomena (a decade occurs always after 10 pulses)?


1 Answer 1


It is best to use the second counter with a synchronous design and share the same clock between both counters.

Use the decoded "decade pulse" as a count enable of the second counter.

Use the decoded "decade pulse" as a synchronous load back to zero of the first counter as opposed to connecting it to an asynchronous clear input.

The reason these steps are very important is because it is possible that the output of the decade decode AND gate output could have glitches on its output as the counter outputs change state. Using that output as a clock could result in the glitches giving extra clock pulses.

  • \$\begingroup\$ Suppose we build counter B with synchronous design using some T flip-flops. So far you have proposed me to drive Y to the input of the T flip-flop of the LSB. However is the pulse Y that is produced by Y enough to make the input go high and tick the clock once simultaneously? Or it is too short? \$\endgroup\$
    – bolzano
    Sep 10, 2016 at 12:26
  • \$\begingroup\$ The clock should ideally be a 50% duty cycle waveform derived from a source separate from the two counter circuits and then used to clock ALL the flip-flops in the the counters. Your choice to select the flop types (D, T or JK). \$\endgroup\$ Sep 10, 2016 at 12:33
  • \$\begingroup\$ If you use the steps I outline the "decade decode" pulse should nominally be a full clock period wide. \$\endgroup\$ Sep 10, 2016 at 12:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.