Asynchronous Counters

Asynchronous Counter: Use cascaded flip-flops to create binary counting sequences.

Up/Down Counter: Digital circuit that can increment up (count up) or down (count down) depending on the control inputs.

• Since it isn’t controlled by a clock signal, it is asynchronous.

Example: 1, 2, and 3-bit up counter

1. Clock
2. One bit Up Counter
3. Two bit Up Counter

Example: 3-bit Up Counter Circuit

Output:

q0: 0 1 0 1 0 1 q1: 0 0 1 1 0 0 a2: 0 0 0 0 1 1

Or, looked at vertically, we can see how it counts up

Step	q1	q2	q3	Numeric
0	0	0	0	0
1	0	0	1	1
2	0	1	0	2
3	0	1	1	3
4	1	0	0	4
5	1	0	1	5

Example: 3-bit Down Counter Circuit

• This is the same design at the Up Counter, we just look at Q’ instead of Q.

Example: Up/Down Counter Part

Example: Frequency Division

• This divides the frequency by 2.
  • Chain to divide by powers of 2. (2, 4, 8, …)
• This is the same behavior as the counter, except we only observe the final output
  • Up or down counter doesn’t matter for frequency division because we only look at the final Q in the chain.

Example: Frequency Multiplication

• This multiplies the frequency by 2.
  • Chain to multiply by powers of 2. (2, 4, 8, …)
• Don’t look at this as logic, look at this as frequencies.
• The buffer functions as a delay
  • The delay doesn’t need to be super precise, though it’ll affect the duration of how long it stays at high and low frequencies.