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A synchronous counter, in contrast to an asynchronous counter, is one whose output bits change state simultaneously, with no ripple. The only way to build such a counter circuit from J-K flip-flops is to connect all the clock inputs together, so that each and every flip-flop receives the exact same clock pulse at the exact same time.
Each of the higher-order flip-flops are made ready to toggle (both J and K inputs “high”) if the Q outputs of all previous flip-flops are “high.” Otherwise, the J and K inputs for that flip-flop will both be “low,” placing it into the “latch” mode where it will maintain its present output state at the next clock pulse.
Since the first flip-flop needs to toggle at every clock pulse, its J and K inputs are connected to Vcc, where they will be “high” all the time. The next flip-flop need only “recognize” that the first flip-flop’s Q output is high to be made ready to toggle, so no AND gate is needed. However, the remaining flip-flops should be made ready to toggle only when all lower-order output bits are “high,” thus the need for AND gates.
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