Digital Teaching Aid
 Counter - Lesson 8
 Lesson Plan
 (introduction...) Introduction Asynchronous counter Synchronous counter Seven segment indicator and decoder

### Asynchronous counter

Fig. 8-1: Asynchronous counter with JK-MS-FF in toggle mode

To understand how this counter works lets have a look at the timing diagram:

Fig. 8-2: Timing diagram, asynchronous mod 8 counter

The frequency of waveform C is one half that at B, but is only one-eighth the clock frequency.

The FF's are negative edge triggered, hence output signals change only at the falling side of the clock pulse.

Fig. 8-3: Truth table, asynchronous mod 8 counter

 CLK transition C B A 0 0 0 0 1 0 0 1 2 0 1 0 3 0 1 1 4 1 0 0 5 1 0 1 6 1 1 0 7 1 1 1 0 0 0 0

A three Flip-Flop counter is often referred to as a mod 8 (modulus 8) counter since it has 8 states.

 23 = 8 output conditions (The exponent equals to the number of Flip-Flops)

The largest decimal number which is represented by a 3 Flip-Flop counter is:

 23 - 1 = 7 In general: 2n - 1

Count Down mode

Switching the clock inputs of each Flip-Flop to the

outputs causes the counting sequence to start at 111 down to 000.

HO: How many Flip-Flops are required to construct a mod-128 counter? A mod-32? What is the largest decimal number that can be stored in a mod-64 counter?

Solution:

* mod-128 must have 7 Flip-Flops (27 = 128)
* mod-32 must have 5 Flip-Flops
* mod-64, the largest decimal number is 63