Applications of Shift Registers

Applications of Shift Registers

• We have seen that primary use of shift register is temporary data storage and bit manipulations. Some of the common applications of shift registers are as discussed below.

1. Delay Line

• A Serial–In–Serial–Out (SISO) shift register can be used to introduce time delay ∆t in digital signals. The time delay can be given as

 ∆t = N × 1/f_c

where N is the number of stages (i.e. flip–flops) and f_c is the clock frequency.

• Thus, an input pulse train appears at the output delayed by ∆t. The amount of delay can be controlled by the clock frequency or by the number of flip–flops in the shift register.

2. Serial–to–Parallel Converter

• A Serial–In–Parallel–Out (SIPO) shift register can be used to convert data in the serial form to the parallel form.

3. Parallel–to–Serial Converter

• A Parallel–In–Serial–Out (PISO) shift register can be used to convert data in the parallel form to the serial form.

4. Shift Register Counters

• A shift register can also be used as a counter. A shift register with the serial output connected back to the serial input is called shift register counter. Because of such a connection, special specified sequences are produced as the output. The most common shift register counters are the ring counter and the Johnson counter.

5. Pseudo–Random Binary Sequence (PRBS) Generator

• Another important application of shift register is a pseudo–random binary sequence generator. Here, suitable feedback is used to generate pseudo–random sequence. The term random here means that the outputs do not cycle through a normal binary count sequence. The term pseudo here refers to the fact that the sequence is not truly random because it does cycle through all possible combinations once every 2ⁿ – 1 clock cycles, where n represents the number of shift register stages (number of flip–flops).

6. Sequence Generator

• The shift register can be used to generate a particular bit pattern repetitively. Fig. 7.5.1 shows the basic block diagram of a sequence generator. Here, left most flip–flop input accept input accept the Serial in serial input and the right most flip–flop gives serial data output. It is important to note that the serial data output signal is connected as a serial data in. On every clock pulse the data shift operation takes place. We get the loaded bit pattern at the serial output in a sequence. Same bit pattern is again loaded in the register since serial output is connected serial in of the register. Thus, the circuit generates a particular bit pattern repetitively.

7. Sequence Detector

• The shift register can be used to detect the desired sequence. The detection process requires two registers : one register stores the bit pattern to be detected i.e. R₁ and other register accepts the input data stream i.e. R₂. Input data stream enters a shift register as serial data in and leaves as serial out. In every clock cycle, bit–wise comparisons of these two registers are done using EX–NOR gates as shown in Fig. 7.5.2. We know that, the two–input EX–NOR gate gives logic high output when both inputs are either low or high, i.e. when both are equal. When outputs of all the EX–NORS gates are logic high we can say that all bits are matched and hence the desired bit pattern is detected. The final output which indicates that the pattern is detected is taken from four–input AND gate.

• The 4–bit sequence detector shown in Fig. 7.5.2 can be made programmable by loading the desired 4–bit data in the register R₂.

Review Question

1. Explain the applications of shift registers.