Flip-Flops are the basic memory elements in sequential circuits. In ECET exams, questions from Flip-Flops are guaranteed — especially characteristic tables, excitation tables, and conversions.
This session covers all Flip-Flops, behavior, formulas, and conversion rules with examples.
📘 Concept Notes – Flip-Flops
A Flip-Flop stores 1 bit of data. It changes state on:
- Clock signal
- Inputs (J, K, T, D, S, R)
🔄 1. SR Flip-Flop
Inputs: S (Set), R (Reset)
Characteristic Equation:
Characteristic Table:
| S | R | Q(next) |
|---|---|---|
| 0 | 0 | Q |
| 0 | 1 | 0 |
| 1 | 0 | 1 |
| 1 | 1 | Invalid |
🔄 2. JK Flip-Flop
Characteristic Equation:
Characteristic Table:
| J | K | Q(next) |
|---|---|---|
| 0 | 0 | Q |
| 0 | 1 | 0 |
| 1 | 0 | 1 |
| 1 | 1 | Toggle (Q’) |
🔄 3. D (Data) Flip-Flop
Characteristic Equation:
Characteristic Table:
| D | Q(next) |
|---|---|
| 0 | 0 |
| 1 | 1 |
🔄 4. T (Toggle) Flip-Flop
Characteristic Equation:
Characteristic Table:
| T | Q(next) |
|---|---|
| 0 | Q |
| 1 | Q’ |
🧠 EXCITATION TABLES (Used for Conversion)
SR Excitation Table:
| Q | Q(next) | S | R |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 0 | 0 |
JK Excitation Table:
| Q | Q(next) | J | K |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
D Excitation Table:
| Q | Q(next) | D |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
T Excitation Table:
| Q | Q(next) | T |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
⚙️ Flip-Flop Conversions
Conversion means:
➡️ Given one FF at input
➡️ Design it to behave like another FF
General Steps:
- Draw required FF truth table
- Use excitation table of available FF
- Derive boolean equations for inputs
🔁 Example – Convert JK Flip-Flop to D Flip-Flop
We want:
Using JK excitation table:
| Q | D (Qnext) | J | K |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
Derive expressions:
For J:
For K:
✔️ Final Converted Design:
- J = D
- K = D’
🔁 Example – Convert D Flip-Flop to T Flip-Flop
We want:
From D FF equation:
✔️ Final:
D = T XOR Q
🔁 Example – Convert SR to JK
Use SR excitation + JK desired output.
Final results (standard):
🔖 Summary of Standard Conversion Formulas
| From → To | Formula |
|---|---|
| D → T |  |
| D → JK |  |
| JK → D |  |
| T → D | |
| SR → JK |  |
🔟 10 MCQs – ECET 2026
Q1. Flip-Flop is a ___ element.
A) combinational
B) sequential
C) arithmetic
D) logical
Q2. Characteristic equation of JK FF?
A) Qnext = D
B) Qnext = T ⊕ Q
C) JQ’ + K’Q
D) S + R’Q
Q3. D Flip-Flop eliminates the problem of:
A) Toggle
B) Race around
C) Meta-stability
D) Delay
Q4. T FF toggles when:
A) T=0
B) T=1
C) T=Q
D) T=Q’
Q5. In D→JK conversion, J equals:
A) D’
B) D
C) 1
D) Q
Q6. SR FF invalid state occurs when:
A) S=0, R=0
B) S=1, R=0
C) S=1, R=1
D) S=0, R=1
Q7. Flip-Flops are clocked:
A) on positive/negative edges
B) randomly
C) analog signals
D) none
Q8. T FF characteristic equation is:
A) Qnext = D
B) Qnext = Q
C) Qnext = T ⊕ Q
D) Qnext = JQ’
Q9. To convert JK to D:
A) D = JQ
B) D = KQ’
C) D = JQ’ + K’Q
D) D = J + K
Q10. Flip-Flops are made using:
A) Gates only
B) Multiplexers
C) Latches + Clock
D) Registers
✅ Answer Key
| Q.No | Answer |
|---|---|
| Q1 | B |
| Q2 | C |
| Q3 | B |
| Q4 | B |
| Q5 | B |
| Q6 | C |
| Q7 | A |
| Q8 | C |
| Q9 | C |
| Q10 | C |
🧠 Explanations
- Q1 → B: FF stores memory → sequential
- Q2 → C: JK eqn = JQ’ + K’Q
- Q3 → B: D FF avoids race around
- Q4 → B: T=1 → Toggle
- Q5 → B: JK = D, D’
- Q6 → C: SR invalid = 11
- Q7 → A: Edge-triggered clock
- Q8 → C: T FF toggles on XOR
- Q9 → C: Standard derived result
- Q10 → C: FF = Latch + Clock
🎯 Why This Topic Matters
- Flip-Flops & conversions are guaranteed ECET questions.
- Very scoring and concept-based.
- Needed for counters, registers, FSMs.
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