0
\$\begingroup\$

Asynchronous counters can not work at high clock frequencies and cause problems with decoding circuits. What does this mean in terms of timing and operation of the circuit?

Why a “re-synchronizing” output flip-flop may be required for these counters?

\$\endgroup\$

2 Answers 2

1
\$\begingroup\$

The problem with asynchronous counters is that the clock of each flip flop comes from the preceding state, so the propagation delays of the clocks get accumulated and each bit changes at a slightly different time than the others. You need to resynchronize the outputs to make them change together at the same time. Otherwise the combinatorial logic after the counter will take a long time to settle and use a lot of power during that stormy period unnecessarily.

\$\endgroup\$
1
\$\begingroup\$

Made from sufficiently fast gates, asynchronous counters count perfectly, even at high frequencies. Its just that you will have some difficulties reading the current count.

Each more significant bit changes later than its preceding less significant bit. Given enough bits, and enough frequency, it can happen that the LSB update of the next clock happens before the MSB update of the previous clock. In this case, at no time the but pattern at the output is guaranteed to match the count - but if you stop clocking, the bits will eventually settle to the right pattern, so I am hesitant to claim that the counter didn't work, i.e. count, the whole time.

When working at low frequencies, you can decode the counter output of an asynchronous counter using conventional logic, it will "just" produce erratic results for the short time the counter "rattles the bits". If you ignore this glitch on the output, everything works. With increasing frequency, the "short glitch" starts becoming longer compared to the clock period, and at some point you can't just "ignore the minor glitch" anymore. That's where the latch helps: You delay passing the counter output to the next stage until all bits have settled, and pass them all at once then.

Obviously, in the extreme case depicted in the beginning of the answer, where there is no point in time you get all bits right, a simple latch can't help anymore, but multiple staggered latches might do the job.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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