The NAND Flip-Flop

This is a variation on the previous flip-flop circuit. It uses a pair of cross-coupled NAND gates to form another basic flip-flop.

Click on the S and R input buttons and see what the output does.
The inputs are normally Hi and change to Lo only as long as you hold the mouse button down. When you release it the input returns to Hi. Notice that the outputs change as soon as an input goes Lo.

R S flip-flop

Compare this with the previous circuit. The key differences revolve around the inputs. One difference is that the "remembering" condition occurs when both inputs are Hi at the same time. With the previous circuit it occurred when both were Lo. The circuit is SET and RESET with momentary Lo inputs instead of momentary Hi inputs.
The other difference is the labelling of the inputs. Notice that the SET input is now at the top, opposite the Q terminal. This is a different location from the NOR flip-flop. This circuit still suffers from the indeterminate problem that arises when both inputs are Lo and then go Hi at exactly the same time. Again, it is up to the designer to prevent this from happening.
When is this circuit used? Again, that is up to the designer. For situations where inputs are normally Hi and go Lo only when some event occurs, use this NAND circuit. For situations where the inputs are normally Lo, use the NOR circuit.

For a typical NAND flip-flop application, click here (use back to return).

Back to Basic Logic Gates.

Next - Standard Symbols