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Counters•Counters are a specific type of

sequential circuit.•Like registers, the state, or the flip-

flop values themselves, serves as the “output.”

•The output value increases by one on each clock cycle.

•After the largest value, the output “wraps around” back to 0.

Using two bits:

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Present State Next State A B A B 0 0 0 1 0 1 1 0 1 0 1 1 1 1 0 0

00 01

1011

1

11

1

(ASYNCHRONOUS COUNTER)

RIPPLE COUNTER

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Asynchronous Counters

• This counter is called asynchronous because not all flip flops are hooked to the same clock.

• This is called as a ripple counter due to the way the FFs respond one after another in a kind of rippling effect.

•In a ripple counter, a flip-flop output transition serves as a source for triggering other flip-flops.

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Asynchronous counters

If the clock has period T. Q0 has period 2T. Q1 period is 4TWith n flip flops the period is 2n.

• Binary Ripple Counter–(2 bits, 3 bits, 4 bits, …)– 1 to 0

•Binary Countdown Counter– 0 to 1

•BCD Ripple Counter–4 bits

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BINARY RIPPLE COUNTER

10

BINARY COUNTDOWN COUNTER

11

12

BCD RIPPLE COUNTER

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