digital counter circuits

Posted on Feb 6, 2014

The circuit in Figure 1 is that of a 4-bit asynchronous counter, also known as a `ripple counter`. It consists of 4 J-K flip-flops whose J and K inputs are tied to logic `1`. This connection causes the output of each J-K flip-flop to toggle every time it gets a clock pulse. By using the output of the previous flip-flop to clock the next flip-flop,

digital counter circuits
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a `rippling` effect is achieved causing the flip-flops to count in binary fashion. The circuit in Figure 2 is that of a 4-bit synchronous counter. It consists of 4 J-K flip-flops, all of which are clocked at the same time, hence the name `synchronous counter`. Thus, the toggling of the output of the flip-flops in this counter depends on the states of their J and K inputs. Recall that the J-K flip-flop`s output will only toggle when both J and K are `high`. The first flip-flop (lease-significant bit) of a synchronous counter has its J and K inputs directly tied to logic `1`. This causes its output to toggle automatically every time it gets a clock pulse. The second flip-flop`s J and K inputs are directly tied to the output of the first flip-flop. Thus, even if they get clocked at the same time, the second flip-flop will only toggle half the times as the first one. The subsequent flip-flops` J and K inputs are tied to an AND gate whose inputs are tied to the outputs of the last two flip-flops before it. This ensures that each of these flip-flops will toggle at half the rate as the flip-flop before it, even if they are all clocked at the same time.

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