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I'm trying to build a Modulus 5 counter using TTL in Multisim that will count 0-4 and reset on 5. From the K-map I got Q2' + Q0' and using that...

Strobe = Q2'+Q0' Strobe''= (Q2'+Q0')'' = (Q2'*Q0')'' Strobe = (Q2Q0)'

  • ' = not.

I've built the circuit in Multisim as shown: enter image description here

However, it stays at 0. Can someone help me figure out what's wrong?

Licentia
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    `count 0-4 and reset on 5` ... how can it reset on 5 if it only counts to 4? – jsotola Nov 05 '22 at 00:49
  • Licentia - Hi, I notice that you commented to say "That worked" below an answer. If your question has been solved, please consider "[áccepting](https://meta.stackexchange.com/q/5234)" what is the "best" answer for you (i.e. click the "tick mark" next to that answer, to turn its tick mark green). This shows that you don't need more help and future readers can quickly see which was your choice of answer / solution. More about "áccepting" [here](/help/someone-answers). While it isn't *required*, it is strongly encouraged to áccept the best answer (in your opinion) that solves your problem. Thanks – SamGibson Nov 05 '22 at 13:06

1 Answers1

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If all flipflops are cleared (zero), their inverted outputs ("~1Q") will all be high. This means the inputs to U7A are both high, making its output low, and clearing all of the flipflops. The circuit is therefore locked up - if it's cleared, it keeps clearing itself, with no way out of the cleared state.

You should connect the inputs of U7A to the non-inverted ("1Q") outputs of the flipflops instead, as you've actually already figured out: Strobe = (Q2Q0)'

Note that an asynchronous circuit such as yours might misbehave due to glitches since gates and flipflops don't switch instantly. Synchronous logic (where all flipflops are driven by the same clock signal) is much more robust and generally preferred.

Jonathan S.
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    I don't know how I missed that. That worked. Thanks! – Licentia Nov 05 '22 at 01:33
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    Elaborating on the last paragraph of the correct answer from @Jonathan S. : as a general rule you should never use the asynchronous set and clear inputs on flip-flops, counters, etc., Taking it a step further, you should only use edge-triggered ICs. There still are some pulse triggered flip flops (also known as ones-catching) available that shouldn't have been used in new designs since the early 1970s. Lastly, you'll usually find it easier to use positive edge-triggered IC's: there are more choices and it's easier to connect them all to the same clock. – stretch Nov 05 '22 at 12:34