Question

What would be the maximum operating frequency of Mod-256 counter constructed with JK flip flops, having a propagation delay of 8ns each?

1. 16.525 MHz
2. 15.625 MHz
3. 25.625 MHz
4. 18.525 MHz

Answer 1

Correct Answer - Option 2 : 15.625 MHz

Concept:

Number of flip flops

For mod N counter a total number of flip-flops required is given by:

2ⁿ ≥ N

n: Total number of flip flops.

Maximum frequency

Let ‘n’ number of flip flops are used in a counter having the delay as ‘t_pd’ then the total maximum frequency is defined by:

\(f_{max} = \frac{1}{n \times {t_{pd}}}\)

Calculation:

Given modulus of the counter is 8 and delay of each flip flop is 8 ns

The number of flip flops required is 8 (∵ 2⁸ = 256)

The maximum frequency will be:

\({f_{max}} = \frac{1}{{8 \times 8ns}}\)

\({f_{max}} = \frac{1}{{64 \times {{10}^{ - 9}}sec}}\)

\({f_{max}} = \frac{{1000}}{{64}}MHz\)

fmax = 15.625 MHz