In this post, we will study the *Propagation Delay* in Ripple Counters. We will also solve a sample *Numerical* problem to illustrate the *effect of propagation delays in a 4-bit asynchronous binary counter*.

## Propagation Delay in Ripple Counters – explained

Ripple counters are the simplest type of binary counters because they require the fewest components to produce a given counting operation.

They have one major **drawback**, which is caused by their basic principle of operation. Each FF is triggered by the transition at the output of the preceding FF, as shown in **Figure.1**

We know that **asynchronous counters** are commonly referred to as **ripple counters**. In **ripple counters**, the effect of the input clock pulse is first “felt” by FF0. This effect cannot get to FF1 immediately because of the **propagation delay** **time ****t**_{pd }through FF0. Then there is the propagation delay through FF1 before FF2 can be triggered. Thus, the effect of an input clock pulse “ripples” through the counter, taking some time, due to propagation delays, to reach the last flip-flop.

This means that the second FF will not respond until a time *t*_{pd} after the first FF receives an active clock transition; the third FF will not respond until a time equal to 2 * *t*_{pd} after that clock transition; and so on. In other words, the propagation delays of the FFs **accumulate** so that the *N*th FF cannot change states until a time equal to *N ** *t*_{pd} after the clock transition occurs. This is illustrated in Figure.2, where the waveforms for a 3-bit asynchronous (ripple-clocked) counter are shown.

### Effect of propagation delays in a 3-bit asynchronous (ripple-clocked) binary counter

Notice that all the three flip-flops in the counter of Figure.1 change state on the leading edge of the fourth clock pulse, CLK4. This ripple clocking effect is shown in Figure.2 for the first four clock pulses, with the propagation delays indicated.

The LOW-to-HIGH transition of *Q*0 occurs one delay time (*t**PLH*) after the positive-going transition of the input clock pulse.

The LOW-to-HIGH transition of *Q*1 occurs one delay time (*t**PLH*) after the positive-going transition of *Q*0.

The LOW-to-HIGH transition of *Q*2 occurs one delay time (*t**PLH*) after the positive-going transition of *Q*1.

As you can see, FF2 is not triggered until two delay times after the positive-going edge of the 4^{th} clock pulse ( CLK4).

Thus, it takes three propagation delay times for the effect of the 4^{th} clock pulse (CLK4), to ripple through the counter and change *Q*2 from LOW to HIGH.

This cumulative delay of an asynchronous counter is a major disadvantage in many applications because it limits the rate at which the counter can be clocked and creates decoding problems.

The **maximum cumulative delay*** (N ** *t*_{pd} ) in a counter must be less than the period of the clock waveform (*T*_{clock}).

That is, for the proper counter operation we need,

*T*_{clock} >= *N ** *t*_{pd} **,** where *N *= the number of FFs.

**i.e., 1/f**_{clock}** >=*** N ** *t*pd **or 1/*** N ** *t*pd >=** f**_{clock}

Thus, f_{clock} should be less than or equal to 1/ N * t_{pd}.

Stated in terms of **input-clock frequency**, the maximum frequency that can be used in an asynchronous counter is given by,

*f*_{max} =1/*N ** *t*_{pd}

For example, a 3-bit ripple counter having FFs with identical *t*_{pd }= 50 ns will have a maximum input frequency

Limit of *f*_{max} =1/3 * 50 ns= 6.67 MHz.

**Summary note:**

We have found out that, as the number of FFs in the ripple counter increases, the total delay will increase and *f*_{max} will be lower.

Also will be able to determine the maximum operating frequency based on the number of FFs and propagation delays.

**Numerical problem illustrating the effect of propagation delays in a 4-bit asynchronous binary counter**

The following numerical problem will illustrate the effect of propagation delays in a 4-bit asynchronous binary counter.

A 4-bit asynchronous binary counter is shown in Figure.3a each D flip-flop is negative edge-triggered and has a propagation delay for 10 nanoseconds (ns). Develop a timing diagram showing the *Q *output of each flip-flop, and determine the total propagation delay time from the triggering edge of a clock pulse until a corresponding change can occur in the state of *Q*3. Also, determine the maximum clock frequency at which the counter can be operated.

**Solution**:

The timing diagram with delays omitted is as shown in Figure.3b.

For the total delay time, the effect of CLK16 must propagate through four flip-flops before *Q*3 changes, so

the total propagation delay time* , t*_{p(tot)} = 4 * 10 ns = **40 ns**

The maximum clock frequency is,

*f*_{max} =1/*t*_{p(tot)}

=1/40 ns = **25 MHz**

The counter should be operated below this frequency to avoid problems due to the propagation delay.

Related Posts

**A 3-Bit Asynchronous Binary Counter in UP counting mode | Up Counter**

**Digital Electronics – Hub of posts**

**Author of this post**

This post is co-authored by *Professor Saraswati Saha*, who is an assistant professor at RCCIIT, a renowned degree engineering college in India. Professor Saha teaches subjects related to digital electronics & microprocessors.