For a homework assignment I need to

Redesign the right-shift register circuit of Figure 12-10 using four D flip-flops with clock enable, four 2-to-1 MUXes, and a single OR gate.

The figure mentioned has three states, No change, Load, and Right shift. The loads are each from different inputs. I have been able to figure out how to do any two of these states without the OR gate, but I can't seem to figure out how to get all three to work. I suspect that I need to somehow stop the clock when I want to achieve the "No change" state.

  • 1
    \$\begingroup\$ Is the broadside load supposed to be synchronous or asynchronous, and where is figure 12-10? \$\endgroup\$
    – EM Fields
    Commented Sep 13, 2014 at 6:05

1 Answer 1


Have a look here:


simulate this circuit – Schematic created using CircuitLab

The clock enable input is not drawn on the flip flops, so I've added a name to the corresponding node. When clock enable is low the FFs do not sample the input, thus you are in "no change" state. That is achieved because clock enable is connected to the OR between SHIFT and LOAD. The STOP input is not used.

When either SHIFT or LOAD is high, at each clock cycle the input is sampled and according to the status of the muxes you can present your input data at parallel in or at shift in. I have also included a parallel out output, that is always active (and always valid), together with a shift in and shift out terminals.

  • \$\begingroup\$ Okay, so the clock enable is an input that allows me to enable/disable the clock. (Sounds really obvious now) Out of curiosity, could an AND of the clock and some input accomplish the same thing? \$\endgroup\$
    – duckbrain
    Commented Sep 13, 2014 at 21:27
  • \$\begingroup\$ @Duckbrain yes and no. clock gating is generally a bad idea, that's why clock enable inputs exist. They are not wired as ands, the circuit inside is a bit more complicated \$\endgroup\$ Commented Sep 13, 2014 at 22:00

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.