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A flip-flop is a sequential digital electronic circuit having two stable states that can be used to store one bit of binary data. Flip-flops are the fundamental building blocks of all memory devices.
Types of Flip-Flops
- S-R Flip-Flop
- J-K Flip-Flop
- D Flip-Flop
- T Flip-Flop
S-R Flip-Flop
This is the simplest flip-flop circuit. It has a set input (S) and a reset input (R). When in this circuit when S is set as active, the output Q would be high and the Q” will be low. If R is set to active then the output Q is low and the Q” is high. Once the outputs are established, the results of the circuit are maintained until S or R get changed, or the power is turned off.
Truth Table of S-R Flip-Flop
| S | R | Q | State |
|---|---|---|---|
| 0 | 0 | 0 | No Change |
| 0 | 1 | 0 | Reset |
| 1 | 0 | 1 | Set |
| 1 | 1 | X |
Characteristics Table of S-R Flip-Flop
| S | R | Q(t) | Q(t+1) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | X |
| 1 | 1 | 1 | X |
Characteristics Equation of S-R Flip-Flop
$$\mathrm{Q(t \: + \: 1) \: = \: S \: + \: R” \: Q(t)}$$
J-K Flip-Flop
Because of the invalid state corresponding to S=R=1 in the SR flip-flop, there is a need of another flip-flop. The JK flip-flop operates with only positive or negative clock transitions. The operation of the JK flip-flop is similar to the SR flip-flop. When the input J and K are different then the output Q takes the value of J at the next clock edge.
When J and K both are low then NO change occurs at the output. If both J and K are high, then at the clock edge, the output will toggle from one state to the other.
Truth Table of JK Flip-Flop
| J | K | Q | State |
|---|---|---|---|
| 0 | 0 | 0 | No Change |
| 0 | 1 | 0 | Reset |
| 1 | 0 | 1 | Set |
| 1 | 1 | Toggles | Toggle |
Characteristics Table of JK Flip-Flop
| J | K | Q(t) | Q(t+1) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 1 |
| 1 | 1 | 1 | 0 |
Characteristics Equation of JK Flip-Flop
$$\mathrm{Q(t \: + \: 1) \: = \: j \: k \: Q(t)” \: + \: K”Q(t)}$$
D Flip-Flop
In a D flip-flop, the output can only be changed at positive or negative clock transitions, and when the inputs changed at other times, the output will remain unaffected. The D flip-flops are generally used for shift-registers and counters. The change in output state of D flip-flop depends upon the active transition of clock. The output (Q) is same as input and changes only at active transition of clock
Truth Table of D Flip-Flop
| D | Q |
|---|---|
| 0 | 0 |
| 1 | 1 |
Characteristics Equation of D Flip-Flops
$$\mathrm{Q(t \: + \: 1) \: = \: D}$$
T Flip-Flop
A T flip-flop (Toggle Flip-flop) is a simplified version of JK flip-flop. The T flop is obtained by connecting the J and K inputs together. The flip-flop has one input terminal and clock input. These flip-flops are said to be T flip-flops because of their ability to toggle the input state. Toggle flip-flops are mostly used in counters.
Truth Table of T Flip-Flop
| T | Q(t) | Q(t+1) |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Characteristics Equation of T Flip-Flop
$$\mathrm{Q(t \: + \: 1) \: = \: T”Q(t) \: + \: TQ(t)” \: = \: T \: \oplus \: Q(t)}$$
Applications of Flip-Flops
- Counters
- Shift Registers
- Storage Registers, etc.
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