# Design of Asynchronous / Ripple counter

The asynchronous or ripple counter consists of series of flip-flops which are not synchronized by the same clock pulse. Design of asynchronous counter involves several steps from selecting the number of flip-flops to drawing the logic circuit diagram.

Before entering the design of the asynchronous counter, you can go through the construction, operation and timing diagram of the asynchronous counter.

## Design steps of asynchronous counter

- Find the number of flip flops using 2
^{n}≥ N, where N is the number of states and n is the number of flip flops. - Choose the type of flip flop.
- Draw the truth table for asynchronous counter.
- Use K-map to derive the flip flop reset input functions.
- Draw the logic circuit diagram.

## Design Problem #1

Design a BCD ripple counter using JK flip flops.

*Step 1: Find the number of flip-flops*

BCD is the 4-bit number and there are 9 valid states in a 4-bit BCD. Hence 4 flip-flops should be used in the design.

The valid states are 0000, 0001, 0010, … 1001.

*Step 2: Choose the type of flip-flop.*

Let us design the BCD ripple / asynchronous counter with JK flip-flops.

*Step 3: Draw the truth table for BCD ripple counter.*

*Step 4: Use Kmap to find the reset logic function.*

The above truth table for BCD counter is implemented in a Karnaugh map to get the reset logic function.

*Step 5: Draw the logic circuit diagram.*

The logic circuit diagram for BCD asynchronous/ripple counter is drawn as below. It is drawn with 4 flip-flops. Since it is an asynchronous counter, the clock pulse is given only to the first flip-flop. For the other flip-flops, the output of the previous flip-flop is given as the clock pulse for the next flip-flop.

The expression obtained from the K-map is drawn using combinational circuits. The output Y is given as RESET for all the flip-flops.

## Design Problem #2

Design a Mod-6 asynchronous counter using JK flip flops.

*Step 1: Find the number of flip-flops*

Mod-6 counter represents that the counter will have 6 states. Thus, N =6.

The number of flip-flops used for counter design is determined using the formula, 2^{n} ≥ N.

By trial and error method, the value of n is found to be 3. That is the number of flip-flops, n = 3.

Hence the 6 counter states are 000, 001, 010, 011, 100, 101.

*Step 2: Choose the type of flip-flop.*

Let us design the mod-6 asynchronous counter with JK flip-flops.

*Step 3: Draw the truth table for mod-6 asynchronous counter.*

*Step 4: Use Kmap to find the reset logic function.*

The above truth table for Mod-6 counter is implemented in a Karnaugh map to get the reset logic function.

*Step 5: Draw the logic circuit diagram.*

The logic circuit diagram for mod-6 asynchronous/ripple counter is drawn as below. It is drawn with 3 flip-flops. Since it is an asynchronous counter, the clock pulse is given only to the first flip-flop. For the other flip-flops, the output of the previous flip-flop is given as the clock pulse for the next flip-flop.

The expression obtained from the K-map is drawn using combinational circuits. The output Y is given as RESET for all the flip-flops.

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