Question

A 4-bit ripple counter and a 4-bit synchronous counter are made by flip -flops having a propagation delay of 10 ns each. If the worst case delay in the ripple counter and the synchronous counter be R and S respectively then

• A. $R=10ns,S=40ns$
• B. $R=40ns,S=10ns$
• C. $R=10ns,S=30ns$
• D. $R=30ns,S=10ns$

Answer 1

The correct option is B $R=40ns,S=10ns$

In ripple counter, total delay time is
$T_e=n{t}_d$
where, $n=$ number of bits
Here, $n=4$
and $t_d=10ns$ (delay of each FF)
So, we get the worst case delay in ripple counter as
$T_e=4\times 10ns=40ns$
(or) $R=40ns$
In synchronous counter, all FF's are clocked simultaneously. So its worst delay will be equal to delay of one FF, i.e.,
$T_c=10ns$
(or) $S=10ns$