Page 60 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

They are also known as twisted ring counters. The logic circuit given below shows a Johnson Counter. The circuit
consists of four D flip-flops which are connected.

Clock pulse Q1 Q2 Q3 Q4

0 0 0 0 1

1 0 0 0 0

2 1 0 0 0

3 1 1 0 0

4 1 1 1 0

5 1 1 1 1

6 0 1 1 1

7 0 0 1 1

Applications of Shift Registers
• The shift registers are used for temporary data storage.

• The shift registers are also used for data transfer and data manipulation.
• The serial-in serial-out and parallel-in parallel-out shift registers are used to produce time delay to digital circuits.
• The serial-in parallel-out shift register is used to convert serial data into parallel data thus they are used in
communication lines where demultiplexing of a data line into several parallel lines is required.
• A Parallel in Serial out shift register is used to convert parallel data to serial data.
An n-stage Johnson counter yields a count sequence of 2n different states, thus also known as a mod-2n counter.
Since the circuit consists of four flip-flops the data pattern will repeat every eight clock pulses as shown in the truth
table. The main advantage of the Johnson counter is that it only needs n number of flip-flops compared to the ring
counter to circulate a given data to generate a sequence of 2n states.

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CITS : E & H - Electronics Mechanic - Lesson 9 - 29