Question

Design a modulo-3 binary counter using SR flip-flops.

Answer 1

So here we will design modulo-3 binary counter using SR flip flops.

Steps to design MOD-3 synchronous counter-

1. Decide the number of flip flops and type of flip flop.

We have to design a Mod-3 counter, therefore it will have 3 states i.e. 00,01,10,

Number of flip flops required = 2 (mod 3 counter is given, so no. of flip flops= 3-1, that is 2)

Type of flip flop = SR flip flop (given)

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2. Make the excitation table of SR flip flop.

EXCITATION TABLE OF SR FLIP FLOP

= Present state

= Next state

S and R are inputs of flip flop

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3. Form state diagram.

Our counter's initial and final state is 00. counting will happen as given below-

00->01

01->10

10->00

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4. Draw circuit excitation table.

= INITIAL STATE OF FLIP FLOP 1

= FINAL STATE OF FLIP FLOP 1

= INITIAL STATE OF FLIP FLOP 2

= FINAL STATE OF FLIP FLOP 2

AND = INPUTS OF FLIP FLOP 2

AND = INPUTS OF FLIP FLOP 1

There will only 3 states as it is mod-3 counter .

Here, we have use SR excitation table and state diagram to form circuit excitation table.

o>(Row 1) - Present state of counter (initial)=00 then it's next state will be 01.

o>(Row 2) - Now present state of counter = 01 then it's next state will be 10.

o>(Row 3) - Present state of counter =10 then it's next state will be 00.

• The values of   and are used to find values of   and using excitation table.
• The values of and are used to find values of   and using excitation table.

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5. Obtain simplified equations using k-map.

Here, we have used k- map to get simplified equations of , , , in terms of, , ,.

, =

=

=.

=

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6. Draw logic diagram.

• Two flip flops and clock(positive edge triggered) is used.
• We have implemented simplified equations in step 5 here in logic diagram.
• We will get output from  , .

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