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CHAPTER 8

Counter Circuits

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CLASSIFICATION OF COUNTERS Asynchronous (ripple)

They use the O/P of one FF to generate the clock transition on another FF (s)

Synchronous clock inputs on each FF are connected

together

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CLASSIFICATION OF COUNTERS Binary

0,1,2, ….,2n –1 i- 0,1,2,3 22 states ii- 0,1,…,15 24 states

Decimal 0,1,2, …, 10n ‑ 1

i- 0,1, …. 9 10 states ii- 0,1, …99 100 states

Octal 0,1,2, .. , 8n-1

Special Any specified sequence sf states

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CLASSIFICATION OF COUNTERS up down up/down

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3-bit Asynchronous Binary counter : (Mod-8)

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Count sequenceQ2 Q1 Q0

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

Q0 toggles at each negative edge of the clock input.Q1 toggles at each negative edge of Q0

Q2 toggles at each negative edge of Q1

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Timing diagram of up counters

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DOWN COUNTERS

•To form a down – counter simply take the binary outputs from the Q’ outputs instead of the Q outputs

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Timing diagram of down counters

• We can alternatively get count-down counter by connecting Q’ of each stage to the negative edge triggered clock pulse of the next stage and get the output from Q output of the flip-flops

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DESIGN OF DIVIDE – BY – N COUNTERS

the frequency of the 22 output line is one-eighth the frequency of the input clock.

So, a MOD-8 counter can be used as a divide–by–8 frequency divider

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A MOD-5 Ripple Binary Counter

the number 5 will appear at the outputs for a short duration, just long enough to Reset the flip-flops. The resulting short pulse on the 20 line is called a glitch.

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Timing Diagram of MOD-5 Ripple Binary Counter

Any modulus counter (divide – by – N counter) can be formed by using external gating to Reset at a predetermined number.

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BCD RIPPLE (DECADE) COUNTER Counter with ten states in their sequence

(modulus –10) are called decade counters.

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3-decade decimal BCD counter

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SYNCHRONOUS COUNTERS

COUNT SEQUENCE FLIP-FLOPS INPUTS A3 A2 A1 A0 TA3 TA2 TA1 TA0 0 0 0 0 0 0 0 0 1 1 . . 1

0 0 0 0 1 1 1 1 0 0 . . 1

0 0 1 1 0 0 1 1 0 0 . . 1

0 1 0 1 0 1 0 1 0 1 . . 1

0 0 0 0 0 0 0 1 0 0 . . 1

0 0 0 1 0 0 0 1 0 0 . . 1

0 1 0 1 0 1 0 1 0 1 . . 1

1 1 1 1 1 1 1 1 1 1 . . 1

TA0 = 1

TA1 = A0

TA2 = A0 A1

TA3 = A0 A1 A2

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SYNCHRONOUS COUNTERS We can conclude from the excitation table (using

a Karnauph map or by inspection that TA0 = 1 TA1 = A0 TA2 = A0 A1 TA3 = A0 A1 A2

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Synchronous Counter Using J-K

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SYNCHRONOUS BINARY DOWN-COUNTER The only change is that the Q outputs are used as inputs to

the T (or J–K) input of the next flip-flop.

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UP/DOWN SYNCHRONOUS COUNTERS