# Is my understanding on flip flops accurate?

I have done a lot of research on flip flops so to speak and I have put together my understanding, please correct were I’m wrong.

• Flip flops are bistable multivibrators able to store two states (one and zero), indefinitely as long as current is flowing through the circuit, and can only change their output when triggered by an external input.

• Flip flops are made using a feedback loop.

• NOR and NAND gates are not the only gates used in a flip flip, they are preferred because they are less expensive, easier to design and use fewer transistors/resources.

• The output of the NAND/NOR flip flop should invert (Q = notQ) to provide certainty.

For the last point I mentioned, I did cause of, when a basic flip flop is drawn, using NAND gates, it is usually stated that the both outputs should not be the same,in this case(both Q’s != 1) and they use a D FF to eliminate that from happening, so after much asking why I gathered that, it doesn’t cause any damage, it kinds of just brings about certainty or uniformity, that at all times you know( Q = not Q), is this correct?

• what do you mean by the last point? Commented Jun 17, 2022 at 22:01
• Those statements seem to be correct, except that I wouldn’t describe a flip flop as a multivibrator, although others may differ.
– Frog
Commented Jun 17, 2022 at 22:02
• When the word "multivibrator" was used (long ago) there were three types: astable, bistable and monostable, for oscillators, RS flip flops, and one shots, respectively. A lot of the 555 documentation uses "monostable" without "multivibrator," interchangeably with "one-shot." Commented Jun 17, 2022 at 22:28
• Sometimes an internal RS flip flop on a more complicated flip flop - 7474, e.g. will have both outputs high. The external outputs of the IC will be complementary, though. I don't know how you could make a flip flop without NAND, NOR gates. Using an inverter with AND/OR gates, if that's what you mean, is just making a NAND/NOR gate. Commented Jun 17, 2022 at 22:40
• @jsotola, for the NAND/NOR FF it’s not a must for their output to invert(Q =notQ) or complement each other and should not reach an “invalid” state but it’s advised that they invert, for certainty, or uniformity across designs maybe. Commented Jun 18, 2022 at 1:47

"indefinitely as long as current is flowing through the circuit" -- it is not that current is flowing in the circuit, but that a supply voltage is applied (CMOS logic circuits generally do not operate on the basis of current, but on voltage).

"Flip flops are made using a feedback loop" yes; this (positive feedback) is what allows a FF to 'remember' its state.

"The output of the NAND/NOR flip flop should invert (Q = notQ) to provide certainty.". The common definition of notQ is the inverse of Q. However having a notQ output is not required for a FF.

Basically a FF is a memory -- when the set/reset inputs return to (typically) 0, the FF remembers which one was high most recently. Thus with fixed inputs (0,0), the output can be in one of two states. This is a memory.

• Thank you so much for this answer, for the last one I mentioned it cause of, when the basic flip flop is drawn, using a NAND, they state that the both outputs should not be the same in this case(both Q’s = 1) and they use a D FF to eliminate that from happening, so after much asking why I gathered that, it doesn’t cause any damage, it kinds of just brings about certainty or uniformity, that at all times you know( Q = not Q), is this correct? Commented Jun 18, 2022 at 2:21

You can build a static storage element out of any circuit that provides positive feedback to hold state.

What you describe, using a pair of NOR or NAND gates, is more accurately called an R-S flip-flop or a bistable latch. It too uses positive feedback to achieve a stable state, achieved by two inversions in a row.

A common method used in chips nowadays is to construct a D latch using a pass transistor 2:1 multiplexer. The multiplexer selects ‘hold’ or ‘follow input’ behavior for the output. A pair of these latches are used to construct a D type register flip-flop.

Static RAM memories will use specialty cells constructed from FETs (typically 6 or 4) for a single bit of storage. Internally the cells use feedback to keep state. The RAM write logic selects a group of cells for access, which are then set or cleared depending on the write data bit states.

• Thank you for your answers, please I don’t really understand this, 1) are you saying NAND/NOR FF are the accurate type of Flip Flops 2) is the D FF not built because the Q outputs have to be the compliment of each other? Commented Jun 18, 2022 at 20:31
• I think you are being too narrow in your thinking about what is a flip-flop. These are many, many ways to make a flip-flop, but they all share the common trait of using positive feedback to store state. Commented Jun 18, 2022 at 20:54
• Yes I know that there are many ways to make a Flip flop, not just using the NAND/NOR flip flop, they may be preferred for reasons I stated above. But want I want to know, let’s take the NAND flip flop as an example, do their outputs invert becuase of uniformity/certainty in designs? Commented Jun 18, 2022 at 21:07
• Look at the truth table for the NOR or NAND flip-flop. The inversion provides a means for the flop to change state based on its inputs, regardless of what state the output is in. This is impossible without the inversion: cross-coupled OR or AND don’t work because they can get stuck at 1 (OR) or 0 (AND). Commented Jun 20, 2022 at 21:27
• When you say there are many ways to make a flip flop, please can you give examples of the many ways? Commented Jun 20, 2022 at 21:45