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After simulating the following circuit in Multisim :enter image description here

I got these timing diagrams (1 ms/Div and 5 V/Div) :

enter image description here

I think that the output Q is inconsistent , shouldn't it look like this instead ? :

enter image description here

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The big problem is that it could be either waveform - the data needs to be setup prior to clocking the 7474 flip flop. It is a positive edge driven device and if you setup data at the instant it receives a positive clock edge then you cannot guarantee what the data is. The fact that your simulator model decides to assume the data is high when the clock rises and is low on the next clock edge is irrelevant because you need to drive the flip flop correctly or you'll get garbage in real life and lies in the simulator.

Setup and hold times for a flip flop: -

enter image description here

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  • \$\begingroup\$ are you actually referring to the hold time ? because according to 7474 datasheet it's around 5 ns so this might be the cause of the problem ,isn'it ? \$\endgroup\$ – Hilbert Dec 14 '18 at 15:43
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    \$\begingroup\$ @Hilbert no, I'm referring to exactly what I said, the setup time. The hold time is hardly a problem in this design because the data is held for nearly 1 ms after clock has risen. Setup time (20 ns) means the data MUST BE present and stable 20 ns before the rising clock edge. \$\endgroup\$ – Andy aka Dec 14 '18 at 15:48
  • \$\begingroup\$ But according to the timing diagrams above we can see that the data is also stable before the rising edge for 1 ms or did I misunderstood something ? link \$\endgroup\$ – Hilbert Dec 14 '18 at 16:03
  • \$\begingroup\$ The data needs to be stable prior to the rising clock edge. All your digrams show is that the data changes at the rising clock edge and that is the problem. I cannot tell from those diagrams if the data is stable for at least 20 ns prior to the rising clock edge. Can you? \$\endgroup\$ – Andy aka Dec 14 '18 at 16:06
  • \$\begingroup\$ Could you please the link in my previous comment ? \$\endgroup\$ – Hilbert Dec 14 '18 at 16:10

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