Objective: Design, construct, and test a three-bit counter that
counts up or down. An enable input E determines whether the counter
is on or off.
If E=0, the counter is disabled and remains at its present
count even though clock pulses are applied to the flip-flops.
If E=1, the counter is enabled and a second input, x,
determines the direction of the count.
If x=1, the circuit counts upward with the sequence 000, 001,
010, 011, 100,...