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Normally, for any counter, we can use both PET (Positive edge triggering) or NET (Negative edge triggering). But, most of the time we use NET. The reason is that in case of PET, considering a standard asynchronous counter ( n-bit), the output of nth flip-flop will change it's state, as it's preceding flip-flop changes it's state from 0 to 1 or low to high. In general, we take the output of flip-flop from Q input, and, the counter will seem to run in reverse order, if PET is used.

This can be understood by an example. Let us take a 2-bit counter with PET J-K Flip-Flops. Let the clock input is applied to the first Flip-Flop. Now taking a look at it's states. Assume that, initially all the flip-flops are at logic low. At the first rising edge of the clock pulse, FF0 (First flip-flop) will change it's state from 0 to 1. This transition from 0 to 1 by FF0 is the clock input to FF1, which sees a rising edge at it's clock input, will also change it's state from 0 to 1. So, at the first clock pulse, the counter will go from it's initial state of 00 to 11. On, second clock pulse, FF0 will transit from 1 to 0, but FF1 will remain at 1 because it is seeing a falling edge. So, the state will shift from 11 to 10. At third pulse, FF0 will shift from 0 to 1, and FF1 will shift from 1 to 0. so, the state will shift from 10 to 01. Finally, at fourth clock pulse, FF0 will shift it's state from 1 to 0 and, FF1 will remain at it's previous state. so, the new state will be 00. Now, taking a look at complete cycle we will have, 00,11,10,01....... so on. So, it is clear that counter is behaving as down counter or reverse counter.

It is a general practice to take output from Q output and not from Q-bar (~Q). So, a better method is to use NET. Since, the output of FF1 in that case will shift it's state when FF0 will shift from 1 to 0. So, the state cycle in that case will be, 00,01,10,11..... so on. It is clear that output of FF1 is changing only when FF0 is changing from 1 to 0 ie. falling edge.

So, to conclude, we can say that in general, we use negative edge triggering in case of counters.

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