Johnson Counters

Jatinder Kumar chaurasia(11508173)M.Revanth(11503559)O.Purnachandra(11504239)

Introduction to counters• Counters –count no of times a particular event occurred• A counter used two or more flip-flop depending upon the no. of bits

required• There are two types of counters: 1.Asynchronous counter 2. Synchronous counter• Asynchronous counter used a single clock at the first stage• Synchronous counter ,all flip flops are clocked together with a common

clock pulse

Johnson counter

• Johnson counters also called twisted ring counters• It is a modification of ring counter• The only difference is that instead of last output(Qd) back to the

input(Qa) the not of (^Qd) is connected to the first input.

Circuit diagram of 4 bit Johnson counter

Truth table of Johnson counter

Timing diagram of 4 bit Johnson counter

Uses of Johnson counter

• Used in walking LEDs• Used to convert square waves to sine waves

Conclusion

• Johnson counter is a modified ring counter where the output from the last flip-flop is inverted and feed back to the input of first flip-flop

• The main advantage of Johnson counter is that it only needs half the no. of flip flops compared to the standard ring counter of same mod

• If we use n flip-flops then it is known as 2n bit Johnson counter or mod-2n Johnson counter

Components used for making 4 bit Johnson counter• 7474-IC(D-flipflop):2• 5 resistors(100 ohm)• 5 LED bulbs• One battery(9v)• Wires(20m)• One 555 timer(bistable multivibrator used as a clock)

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