Digital Logic and Systems Flashcards

Which sequential circuit element is used to detect a specific binary sequence in a serial data stream?

Answer: Sequence detector (finite state machine)

A finite state machine (sequence detector) is designed to recognize a specific pattern in a serial bit stream by transitioning through defined states.

How many flip-flops are required to build a MOD-16 counter?

Answer: 4

A MOD-16 counter cycles through 16 states (0–15), requiring 4 flip-flops since 2⁴ = 16.

Gray code is preferred over standard binary in mechanical encoders because:

Answer: Only one bit changes between consecutive values

Gray code changes only one bit between consecutive values, preventing erroneous intermediate readings when multiple bits would change simultaneously in binary.

What Boolean expression represents an Exclusive-OR (XOR) gate with inputs A and B?

Answer: A'B + AB'

XOR outputs 1 when the inputs differ: A'B (A=0,B=1) + AB' (A=1,B=0), written as A ⊕ B.

In a 4-to-1 multiplexer, how many select lines are needed?

Answer: 2

A 4-to-1 MUX has 4 data inputs requiring 2 select lines since 2² = 4 possible selections.

Which logic family is known for the highest noise immunity among TTL and CMOS?

Answer: CMOS

CMOS logic has high noise immunity because its switching threshold is approximately at VDD/2, and its output swings rail-to-rail.

A Johnson counter with 4 flip-flops has how many unique states in its sequence?

Answer: 8

A Johnson (twisted ring) counter with n flip-flops produces 2n unique states, so 4 flip-flops give 8 states.