Why This Topic is Important for ECET
Sequential circuits form the core of Digital Electronics because they store and process information based on both current input and past states. In ECET, many questions are framed from flip-flops, counters, shift registers, and state diagrams. Mastering this helps not just in ECET but also in GATE and interviews, as sequential circuits are the basis for memory devices, processors, and embedded systems.
📘 Concept Notes
1. Combinational vs Sequential Circuits
- Combinational Circuits: Output depends only on present inputs. Example: Adders, MUX.
- Sequential Circuits: Output depends on present input + previous states (memory). Example: Flip-Flops, Counters.
2. Building Blocks of Sequential Circuits
(a) Flip-Flops
- Basic memory element that stores 1 bit.
- Types: SR, JK, D, T.
- Flip-flops change state with clock signal.
Example: A D flip-flop with D=1 at clock edge → Q=1 (stores the bit).
(b) Latches
- Level-triggered devices (transparent when enabled).
- Example: SR latch.
(c) Registers
- Group of flip-flops used to store multiple bits.
- Shift registers can shift data left/right.
(d) Counters
- Sequential circuits that count clock pulses.
- Can be asynchronous (ripple) or synchronous.
3. State Machines
- Represent sequential circuit behavior using states + transitions.
- Types: Mealy (output depends on input + state), Moore (output depends only on state).
Example: Traffic light controller is a Moore machine.
⚙️ Formulas
- Number of flip-flops required for a counter:
Frequency division in counter:
States in n flip-flops:
Mod number of counter:
State equation (general form):
🔟 10 MCQs
Q1. A sequential circuit differs from a combinational circuit because it has:
a) Logic gates only
b) Feedback and memory
c) Diodes only
d) None
Q2. How many flip-flops are needed to design a MOD-16 counter?
Q3. A 4-bit ripple counter with input clock of 1 MHz will have output frequency of:
a) 1 MHz
b) 500 kHz
c) 250 kHz
d) 62.5 kHz
Q4. In a JK flip-flop, J=1 and K=1, the output will:
a) Reset
b) Set
c) Toggle
d) Hold
Q5. Which flip-flop is called “universal flip-flop”?
a) SR
b) JK
c) D
d) T
Q6. A shift register can be used for:
a) Multiplication
b) Division
c) Data storage & transfer
d) Amplification
Q7. A 3-bit synchronous counter has how many states?
Q8. The main difference between Mealy and Moore machines is:
a) Clocking method
b) Output dependence
c) Number of states
d) None
Q9. A MOD-10 counter requires how many flip-flops?
Q10. If each flip-flop has propagation delay 20 ns, then total delay of a 5-bit ripple counter is:
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | 4 |
| Q3 | d |
| Q4 | c |
| Q5 | b |
| Q6 | c |
| Q7 | 8 |
| Q8 | b |
| Q9 | 4 |
| Q10 | 100 ns |
🧠 Explanations
- Q1: Sequential circuits use feedback → memory. Correct = b.
- Q2: For MOD-16,
. - Q3: Ripple counter divides clock by . For n=4 →
. - Q4: JK=1,1 → toggle operation. Correct = c.
- Q5: JK flip-flop can perform SR, D, and T functions → universal. Correct = b.
- Q6: Registers store/shift bits, not amplify. Correct = c.
- Q7: Number of states =
. - Q8: In Mealy → output depends on state + input; Moore → only on state. Correct = b.
- Q9: For MOD-10 →
. - Q10: Delay = n × propagation delay = 5 × 20 = 100 ns.
🎯 Motivation / Why Practice Matters
Sequential circuits are high-weightage topics in ECET. You can expect questions on counters, flip-flops, and state machines. By practicing daily:
- You’ll quickly recall formulas during exam.
- You’ll avoid confusion between similar flip-flop behaviors.
- You’ll gain speed in numerical calculations like frequency division, delay, states.
These are direct scoring areas if concepts are clear.
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