A counter with clear is a
sequential circuit
with 1 input and n outputs.
It differs from an ordinary counter in that it has an input that sets the contents
of the counter to 0.
Here is a
state table for a 4-bit counter:
cl o3 o2 o1 o0 | o3' o2' o1' o0'
--------------------------------
0 0 0 0 0 | 0 0 0 1
0 0 0 0 1 | 0 0 1 0
0 0 0 1 0 | 0 0 1 1
0 0 0 1 1 | 0 1 0 0
0 0 1 0 0 | 0 1 0 1
0 0 1 0 1 | 0 1 1 0
0 0 1 1 0 | 0 1 1 1
0 0 1 1 1 | 1 0 0 0
0 1 0 0 0 | 1 0 0 1
0 1 0 0 1 | 1 0 1 0
0 1 0 1 0 | 1 0 1 1
0 1 0 1 1 | 1 1 0 0
0 1 1 0 0 | 1 1 0 1
0 1 1 0 1 | 1 1 1 0
0 1 1 1 0 | 1 1 1 1
0 1 1 1 1 | 0 0 0 0
1 0 0 0 0 | 0 0 0 0
1 0 0 0 1 | 0 0 0 0
1 0 0 1 0 | 0 0 0 0
1 0 0 1 1 | 0 0 0 0
1 0 1 0 0 | 0 0 0 0
1 0 1 0 1 | 0 0 0 0
1 0 1 1 0 | 0 0 0 0
1 0 1 1 1 | 0 0 0 0
1 1 0 0 0 | 0 0 0 0
1 1 0 0 1 | 0 0 0 0
1 1 0 1 0 | 0 0 0 0
1 1 0 1 1 | 0 0 0 0
1 1 1 0 0 | 0 0 0 0
1 1 1 0 1 | 0 0 0 0
1 1 1 1 0 | 0 0 0 0
1 1 1 1 1 | 0 0 0 0
As you can see, our conter behaves like an ordinary one when the
cl signal is 0, and always goes to 0
when the cl signal is 1. |