What is the fundamental difference between combinational and sequential circuits?
Answer: B
Combinational circuits: output = f(present inputs only). Sequential circuits: output depends on present inputs AND past states (memory elements like flip-flops store history)
Q.142Medium
In a synchronous counter with n flip-flops, what is the maximum frequency at which the counter can operate?
Answer: B
In synchronous counters, all flip-flops are clocked simultaneously, so the maximum frequency is limited only by the propagation delay of a single flip-flop, not the cumulative delay.
Q.143Easy
Which of the following is a characteristic of asynchronous counters?
Answer: B
Asynchronous (ripple) counters work by cascading flip-flops where the output of one flip-flop triggers the next, creating a ripple effect.
Q.144Easy
A BCD counter counts from 0 to 9 and then resets. How many flip-flops are minimum required?
Answer: C
BCD (Binary Coded Decimal) requires 4 bits to represent decimal digits 0-9 (2^4 = 16 combinations, but only 10 are used).
Q.145Medium
In a Johnson counter with 4 flip-flops, what is the maximum count sequence length?
Answer: B
A Johnson counter (twisted ring counter) has a sequence length of 2n, where n is the number of flip-flops. For 4 flip-flops: 2×4 = 8 states.
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Q.146Medium
Which logic gate is used in a full adder to generate the carry output?
Answer: C
The carry output of a full adder is: C_out = (A·B) + (B·C_in) + (A·C_in), which requires both AND and OR gates.
Q.147Medium
What is the setup time of a flip-flop?
Answer: B
Setup time (t_su) is the minimum duration that the input signal must be stable and valid before the active clock edge for reliable operation.
Q.148Easy
In a multiplexer (MUX) with 3 select lines, how many input lines can it handle?
Answer: C
A multiplexer with n select lines can handle 2^n input lines. For 3 select lines: 2^3 = 8 input lines.
Q.149Easy
A demultiplexer (DEMUX) with 4 select lines has how many output lines?
Answer: C
A demultiplexer with n select lines has 2^n output lines. For 4 select lines: 2^4 = 16 output lines.
Q.150Medium
In a 3-to-8 decoder, if the enable pin is active low and all select lines are 0, what will be the output?
Answer: C
In a 3-to-8 decoder with active low enable and select lines = 000, only output 0 is activated (becomes 0), while all others remain 1.
Q.151Easy
Which type of flip-flop has the simplest truth table and is used for frequency division?
Answer: B
A T flip-flop toggles its output when T=1 and holds when T=0, making it ideal for frequency division and counter design.
Q.152Medium
In Gray code, how many bits change when transitioning from 5 (0101) to 6 (0110)?
Answer: A
Gray code for 5 is 0111 and for 6 is 0101. Only 1 bit changes between consecutive numbers, which is the main advantage of Gray code.
Q.153Medium
A 32×8 ROM has how many address lines and data lines respectively?
Answer: A
For 32 memory locations, we need log2(32) = 5 address lines. The data width is 8 bits, hence 8 data lines.
Q.154Easy
Which of the following is a volatile memory?
Answer: C
RAM (Random Access Memory) is volatile and loses all data when power is removed. ROM, EPROM, and Flash memory are non-volatile.
Q.155Medium
A 4-bit Ripple Carry Adder has a total propagation delay of 12 ns. If each full adder stage has a propagation delay of 3 ns, what is the delay for carry propagation?
Answer: B
In a 4-bit ripple carry adder, the carry must propagate through 3 stages (from FA1 to FA4), giving delay = 3 × 3 ns = 9 ns.
Q.156Easy
Which of the following is a sequential circuit?
Answer: C
Sequential circuits have memory elements (flip-flops) and their output depends on past inputs. Flip-flops are the basic sequential elements; multiplexers, encoders, and decoders are combinational circuits.
Q.157Hard
In Hamming code for single error correction, if the total number of data bits is 16, how many parity bits are required?
Answer: C
For Hamming code, if 'p' is the number of parity bits, then 2^p ≥ (p + data bits). For 16 data bits: 2^p ≥ p + 16. Testing: 2^5 = 32 ≥ 5 + 16 = 21. ✓
Q.158Medium
What is the main advantage of a Carry Lookahead Adder (CLA) over a Ripple Carry Adder (RCA)?
Answer: B
CLA reduces propagation delay from O(n) to O(log n) by calculating carry signals in parallel, making it much faster than RCA.
Q.159Hard
A SRAM cell requires how many transistors for a 1-bit storage?
Answer: C
A typical SRAM cell uses 6 transistors: 4 for the cross-coupled latch (2 NMOS + 2 PMOS) and 2 access transistors for read/write operations.
Q.160Easy
In state machine design, if a machine has 15 different states, the minimum number of flip-flops needed is?
Answer: B
To represent 15 states, we need n flip-flops such that 2^n ≥ 15. Since 2^4 = 16 ≥ 15, minimum 4 flip-flops are required.
