Q:There are two counters to 16 built from negedge D- FF .
The first circuit is a synchronous one and the second one is a "ripple" (cascading).
Which circuit has less delay?
1 - is ripple counter;
2 - synchronous.
Both circuits consist of 4 FF, synchronous counter also has some logic to control it's operation.
From diagram 3 (for ripple) and 4 (for synchronous) it is seen that the propagation delay of the ripple counter
is 4* t_prop , while the synchronous counter has only 1* t_prop.