Design of Ripple (Asynchronous) Counters

Design of Ripple (Asynchronous) Counters

Steps involved in the design of asynchronous counter

1. Determine the number of flip-flops needed.

2. Choose the type of flip-flops to be used : T or JK. If T flip-flops are used connect T input of all flip-flops to logic 1. If JK flip-flops are used connect both J and K inputs of all flip flops to logic 1. Such connection toggles the flip-flop output on each clock transition.

3. Write the truth table for the counter.

4. Derive the reset logic by K-map simplification.

5. Draw the logic diagram.

Ex. 5.7.1 Design BCD ripple counter using JK flip-flop.

Sol. :

Step 1 : Determine the number of flip-flops needed. The BCD counter goes through states 0-9, i.e. total 10 states. Thus, N = 10 and for 2ⁿ ≥ N, we need n = 4, i.e. 4 flip-flops required.

Step 2 : Type of flip-flops to be used : JK

Step 3 : Write the truth table for the counter

Step 4 : Derive reset logic

Step 5 : Draw logic diagram.

Ex. 5.7.2 Design a 3-bit asynchronous ripple counter using T flip-flops and explain its operation.

AU : CSE : May-09, Marks 16

Sol. : The Fig. 5.7.3 shows a 3-bit asynchronous ripple counter using T flip-flops. As shown in Fig. 5.7.3, the clock input of only first stage flip-flop. The clock input of the second stage flip-flop is triggered by the QA output of the first stage and third stage flip-flop is triggered by the QA output of second stage. Because of the inherent propagation delay time through a flip-flop, a transition of the input clock pulse and a transition of the QA output of previous stage can never occur at exactly the same time. Therefore, flip-flops are never simultaneously triggered, which results in asynchronous counter operation.

Since, T input is connected to logic 1 each flip-flop toggles at clock input. The Fig. 5.7.4 shows the timing diagram for 3-bit asynchronous counter.

Ex. 5.7.3 Design mod 6 ripple counter using T flip-flops.

Sol. :

Step 1 : Determine the number of flip-flop required. Here, counter goes through 0-5 states, i.e., total 6 states. Thus N = 6 and for 2ⁿ ≥ N we need n = 3,

i. e. 3 flip-flops.

Step 2 : Type of flip-flops to be used : T

Step 3 : Write the truth table for counter

Step 5 : Draw logic diagram

Review Questions

1. Explain the working of BCD ripple counter with timing diagrams.

2. Design mod 5 ripple counter using T flip-flops.