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ELECTRONIC DESIGN NOTES - FLIP FLOPS

The topic of logic gates Flip-Flop circuits is presented in this page according to the following simplified structure:

1. NAND FLIP-FLOPS
2. NOR FLIP-FLOPS
3. SR FLIP-FLOPS
4. J-K FLIP-FLOPS
5. D FLIP-FLOPS
6. T FLIP-FLOPS

1. NAND FLIP-FLOP

The problem with NAND FLIP-FLOP is, you just cannot let it take any state. In order to start a NAND FLIP-FLOP, you need to set HIGH both the SET and the RESET pins. The principle is better exemplified if you replace the NAND gate with its equivalent, two OR gates having negated inputs.

The outputs of a NAND Flip-Flop are Q and
Q and they complement each other.

NAND GATES FLIP-FLOP

Schematic Timing Diagram Truth Table Description
 SET RESET Q Q 1 1 0 1 0 1 1 0 1 0 0 1 0 0 0 1
Fig 1: NAND FLIP-FLOP

2. NOR FLIP-FLOP

NOR Flip-Flop is more "user friendly", meaning, you can start it with both the SET and RESET pins being LOW.

NOR FLIP-FLOP

Schematic Timing Diagram Truth Table Description
 SET RESET Q Q 0 0 0 1 1 0 1 0 0 1 0 1 0 0 1 0
Fig 2: NOR gates Flip-Flop

3. SR FLIP-FLOPS

SR Flip-Flop works similar to the NOR Gate Flip-Flop, only it is more complex. First, let's take a look at the SR Flip-Flop truth table.

SR FLIP-FLOP

Schematic  Timing Diagram Truth Table Description
 # SET RESET Q Q Allowed 1 0 0 0 1 yes 2 0 0 1 0 yes 3 0 1 0 1 yes 4 0 1 1 0 unstable 5 1 0 0 1 yes 6 1 0 1 0 yes 7 1 1 0 0 no 8 1 1 0 0 no
Fig 3: SR Flip-Flop

Note positions #7 and #8 in the truth table above: both of them are forbidden, because the outputs do not complement each other. In addition position 8 is unstable, therefore
Q will change its state from 0 to 1 and backwards continuously.

State #4 it is also unstable; in this case Q may be 0, although it shouldn't--in the Truth Table Q is marked as 1.

The point to note is, SR Flip-Flops require a certain sequence of operation, and they need to be protected against accidental situations as are #4, #7, and #8.

4. J-K FLIP-FLOPS

J-K Flip-Flop has a few notable advantages over the SR one. There are no forbidden or indeterminate states, and it complements perfectly well. In addition, J-K Flip-Flops are used in Master-Slave constructions--this topic is explained further in this page.

J-K FLIP-FLOP

Schematic Timing Diagram Truth Table Description
 # SET RESET Q Q Allowed 1 0 0 0 1 yes 2 0 0 1 0 yes 3 0 1 0 1 yes 4 0 1 1 0 yes 5 1 0 0 1 yes 6 1 0 1 0 yes 7 1 1 0 1 yes 8 1 1 1 0 yes
Fig 4: J-K Flip-Flop

The outputs change their state when:
1. the input conditions are met, and
2. on the next falling edge of the clock

5. D FLIP-FLOPS

Undeniably, "D" Flip-Flop it is the easiest one to work with. The name "D" comes from "Data". This Flip-Flop is perfectly stable, and very easy to control. Fact is, D Flip-Flop is just the best!

D FLIP-FLOP

Schematic  Timing Diagram Truth Table Description
 D Q Q 0 0 1 1 1 0
Fig 5: D Flip-Flop
Q
takes the value of D on the next falling clock edge

6. T FLIP-FLOPS

Some applications require that we change Q on each input toggle, without any clock, therefore we can use "T" Flip-Flops for that. However, there are T Flip-Flops built with a clock, and in that case the T Flip-Flop behaves exactly like the D one. Particular to the T Flip-Flop is, it triggers the outputs only when T changes states from 0 to 1.

T FLIP-FLOP

Schematic Timing Diagram Truth Table Description
 T Q Q 0 0 1 1 1 0
Fig 6: T Flip-Flop
Q
takes the value of T

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