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I am currently studying flip-flops.

I understand the purposes of flip-flops, and that they can be made by using NAND or NOR gates.

But what are the reasons for making them these two ways? Does either way work for storing bits? Or is this just for cost consideration?

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  • \$\begingroup\$ Technology availability. Some technologies use NANDs only, some NORs only. Some use both and more.. \$\endgroup\$ – Eugene Sh. Aug 29 '16 at 18:02
  • \$\begingroup\$ They aren't quite then same flip flop circuit. Cross connected NAND gates produce NOT SET, NOT RESET inputs whereas cross connected NOR gates produce SET,RESET inputs. Consider also the different technologies used to make them. Technologies such as relays, valves (tubes for our American colleagues), BJTs etc. NOR gates were simpler to build with valves, BJTs etc. so earlier circuit designs tend to be NOR based. When 7400 series logic came about (TTL) the basic gate was the NAND. Both work. \$\endgroup\$ – JIm Dearden Aug 29 '16 at 18:31
  • \$\begingroup\$ any and all logic can be built solely out of NAND gates. or solely out of NOR gates. the choice of which is an electronics question. if i were stuck on an island with no chips but an unlimited supply of BJT transistors and an assortment of resistors, i know right away how i could create a NOR function out of these discrete electronic parts (that's called "RTL logic"). NAND i think is used with MOS technology. \$\endgroup\$ – robert bristow-johnson Aug 29 '16 at 20:37
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Even though the comments and the answers are quite reasonable, i would like to add some information that i assumed you didn't know based on your question

[CMOS]


Why NAND and NOR gates are the building blocks?

Because all the CMOS circuits are complementary circuits you can`t build a CMOS AND gate, you will end up building a NAND followed by NOT gate to form this AND gate, this is because PMOS is used as pull-ups while NMOS is used as pull-downs. For more information you should look into This question. The figures below shows the CMOS implementation of the NOR,NOT and NAND gates

enter image description here


What is the difference between NAND and NOR versions?

They both do the same logic function, but sometimes one of them is better than the other. Let me give an example

below are the NAND and NOR implementation for this logic function

enter image description here enter image description here

One of the differences here is that A input in the NOR implementation is connected to only two transistors [NOT gate], while in the NAND implementation is connected to 4 transistors [2x NAND gates] which means that in the NOR version the A input have HALF THE CAPACITIVE LOAD than the NAND version

So in this example both versions differ in the loading on A input you might start worrying about this capactive load if for example A is connected to many other circuits

The gist is: depending on many factors one can decide which version is suitable for a circuit but both do the same function

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Well, in practice flip flops are usually built at a transistor level with a highly optimized circuit as they are used all over the place and need to be as small and as efficient as possible. There are a number of ways of building a flip flop depending on the style of logic and the circuit requirements. One method is to use pass transistors and build something that somewhat resembles SRAM cells. Another method is to use dynamic logic where the stored value is actually stored as charge in parasitic gate capacitance, however this results in a minimum clock frequency requirement to continuously refresh the stored value.

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I used to use S-R Flip-Flops made from NOR or NAND gates, depending upon what I was trying to accomplish with each particular design.

The simple way to think of it is this: a FF made from two NAND gates has active LO inputs. A FF made from two NOR gates has active HI inputs.

Ignore the technology behind those gates - this didn't matter for low-speed industrial systems. You get 4- dual-input gates in a 14 or 16 pin package and the goal was to minimise the package count while maintaining extremely-good reliability.

You could get your choice of AND, OR, NAND, NOR, XOR, XNOR. They all cost about the same per package. The goal was to minimise the total number of packages in the design.

Many of my designs were used in industrial settings. I would use FFs made from NAND gates so that the inputs were active LO - this allowed the use of standard "contact closure to ground" signals.

One other advantage to using NAND gates for FFs fed from external inputs: I would use cd4093 CMOS Schmitt-Trigger NAND gates with RC filters on the external inputs. This gave me extremely good noise filtering and the series resistor provided significant ESD and transient protection. Some of those designs were used for decades with no failures.

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both NAND and NOR are universal gates because any circuit can be derived from these. so you can use either one but NOR is the better choice.

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