What is a synchronous counter?

Here we come with a new post on synchronous counter. In my previous post I discussed on asynchronous counter. Now through this post we try to learn some very important topic which create lots of confusion.

• Asynchronous and synchronous counter.
• Single and multimode counters.
• Modulus counters.
• Why we use synchronous counter instead of asynchronous counter?
• Working principal of Synchronous Counter

Asynchronous and synchronous counter

The asynchronous counter is simple and straightforward in operation and construction and usually requires a minimum amount of hardware. In asynchronous counters, each flip-flop is triggered by the previous flip-flop, and hence the speed of operation is limited. In fact, the settling time of the counter is the cumulative sum of the individual settling times of the flip-flops. This type of counters is also called ripple or serial counter. And If all the flip-flops of a counter are clocked synchronously then this is known as a synchronous counter.

Single and multimode counters

The counters can be designed such that the contents of the counter advances by one with each clock pulse; and is said to operate in the count-up mode. The opposite is also possible, when the counter is said to operate in the count-down mode. In both cases the counter is said to be a single mode counter. If the same counter circuit can be operated in both the UP and DOWN modes, it is called a multimode counters.

Modulus counters

Modulus counters are defined based on the number of states they are capable of counting. This type of counter can again be classified into two types: Mod N and MOD < N. For example, if there are n bits then the maximum number counted can be 2n or N. If the counter is so designed that it can count up to 2n or N states, it is called MOD N or MOD 2n counter. On the other hand, if the counter is designed to count sequences less than the maximum value attainable, it is called a MOD < N or MOD < 2n counter.

Why we use synchronous counter instead of asynchronous counter?

If we think on the timing delay to perform an operation of counter then we will find each flip-flop has a specific delay time. In ripple counters these delay times are additive and the total “settling” time for the counter is approximately the product of the delay time of a single flip-flop and the total number of flip-flops.

Now if we want to overcome this problem we need something to do. And the solution is if all the flip-flops are clocked synchronously then we can easily overcome the problem. If all the flip-flops of a counter are clocked synchronously then this is known as a synchronous counter.

Working principal of Synchronous Counter

Synchronous counters can be designed for any count sequence (need not be straight binary). In bellow picture is shown of a simple 4-bit synchronous counter. The above picture is shows A 4-bit synchronous counter with parallel carry. In carefully look that the clock inputs of all the flip-flops are tied together so that the input clock signal may be applied simultaneously to each flip-flop. As we see all flip-flops are T flip-flop and it work on toggle mode when T input is 1. That is why only the LSB flip-flop A has its T input connected permanently to logic 1 (i.e., VCC), while the T inputs of the other flip-flops are driven by some combination of flip-flop outputs. The T input of flip-flop B is connected to the output QA of flip-flop A; the T input of flip-flop C is connected with the AND-operated output of QA and QB. Similarly, the T input of D flip-flop is connected with the AND-operated output of QA, QB, and QC.

As we know a T flip-flop changes its state with the negative transition of each clock pulse for T is high. Flip-flop B changes its state only when the value of QA is 1 and a negative transition of the clock pulse takes place. Similarly, flip-flop C changes its state only when both QA and QB are 1 and a negative edge transition of the clock pulse takes place. In the same manner, the flip-flop D changes its state when QA = QB = QC = 1 and when there is a negative transition at clock input. The count sequence of the counter is given in Table

Counter

You may see that too  Frequency Counter  Asynchronous 3-bit up down counter  Ripple counter  Not Enough, Need More

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