So I am designing a 3-bit asynchronous binary counter which counts up to 11 then repeats using D flip-flops. That much I have done, however I am stuck when it comes to decoding the outputs to stop the counting sequence. I am only supposed to use one logic gate, though. I was thinking of how I could do this using an actual decoder, but I am completely lost when it comes to using just one gate. Any tips to get going? I want to actually figure this out.

  • \$\begingroup\$ How does an asynchronous counter work? It just counts as fast as it can, with no clock input? \$\endgroup\$ – mkeith Nov 18 '15 at 4:25
  • \$\begingroup\$ That's right, there is no clock. \$\endgroup\$ – kgEE19 Nov 18 '15 at 4:27
  • \$\begingroup\$ So the D flip flops have no clock input? \$\endgroup\$ – mkeith Nov 18 '15 at 4:30
  • \$\begingroup\$ They do, but they are connected to the output of the previous flip flop. \$\endgroup\$ – kgEE19 Nov 18 '15 at 4:37
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    \$\begingroup\$ Use the built in schematic editor on this website. \$\endgroup\$ – Asmyldof Nov 18 '15 at 7:36

Can't count to 11 on 3 bits, so I presume you're actually using 4 bits. What you need to do is AND the two highest bits together, and feed that into the async reset of all 4 flip flops. This will reset the counter back to 0 very shortly after it reaches 12.


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