Question

(a) Design a 4-bit ring counter. Use an external asynchronous INIT input to initialize the flip-flops to a valid initial state. Also remember to hook up the CLOCK to all flip-flops.

(b) Design a 4-bit Johnson counter. Use an external asynchronous INIT input to initialize the flip-flops to a valid initial state. Also remember to hook up the CLOCK to all flip-flops.

(c) How many states does the ring counter in part (a) have? How many states does the Johnson counter in part (b) have?

(d) Starting at the initial state, show the repeating counting sequence for both the ring counter in part (a) and the Johnson counter in part (b). (That is, show the state of each counter on a cycle-by-cycle basis until it repeats.)

Answer 1

(a)

When INIT is '0' the 1st flip-flop is preset to '1' and the remaining three flip-flops are cleared to '0'. This gives a valid initial state of 1000.

(b)

When INIT is '0' the 1st flip-flop is preset to '1' and the remaining three flip-flops are cleared to '0'. This gives a valid initial state of 1000.

(c) Ring counter in part (a) has four states.

Johnson counter in part (b) has eight states.

(d)

Ring Counter

Cycle	Q1	Q2	Q3	Q4
1	1	0	0	0
2	0	1	0	0
3	0	0	1	0
4	0	0	0	1
5	Repeat

Johnson Counter

Cycle	Q1	Q2	Q3	Q4
1	1	0	0	0
2	1	1	0	0
3	1	1	1	0
4	1	1	1	1
5	0	1	1	1
6	0	0	1	1
7	0	0	0	1
8	0	0	0	0
9	Repeat