Question

A 4-bit ripple counter consists of flip-flops, which each have a propagation delay from clock to Q output of 15 ns. For the counter to recycle from 1111 to 0000, it takes a total of ________

(a) 15 ns

(b) 30 ns

(c) 45 ns

(d) 60 ns

The question was asked in examination.

My enquiry is from Counter Implementation and Applications topic in chapter Counters of Digital Circuits

Answer 1

Correct option is (d) 60 ns

To elaborate: Since a counter is constructed using flip-flops, therefore, the propagation delay in the counter occurs only due to the flip-flops. Each bit has propagation delay = 15ns. So, 4 bits = 15ns * 4 = 60ns.