Shift Registers

Flip-flop is a 1 bit memory cell which can be used for storing the digital data. To increase the storage capacity in terms of number of bits, we have to use a group of flip-flop. Such a group of flip-flop is known as a Register. The n-bit register will consist of n number of flip-flop and it is capable of storing an n-bit word.

The binary data in a register can be moved within the register from one flip-flop to another. The registers that allow such data transfers are called as shift registers. There are four mode of operations of a shift register.

• Serial Input Serial Output
• Serial Input Parallel Output
• Parallel Input Serial Output
• Parallel Input Parallel Output

Serial Input Serial Output

Let all the flip-flop be initially in the reset condition i.e. Q3 = Q2 = Q1 = Q0 = 0. If an entry of a four bit binary number 1 1 1 1 is made into the register, this number should be applied to Din bit with the LSB bit applied first. The D input of FF-3 i.e. D3 is connected to serial data input Din. Output of FF-3 i.e. Q3 is connected to the input of the next flip-flop i.e. D2 and so on.

Serial Input Parallel Output

In such types of operations, the data is entered serially and taken out in parallel fashion.

Data is loaded bit by bit. The outputs are disabled as long as the data is loading.

As soon as the data loading gets completed, all the flip-flops contain their required data, the outputs are enabled so that all the loaded data is made available over all the output lines at the same time.

4 clock cycles are required to load a four bit word. Hence the speed of operation of SIPO mode is same as that of SISO mode.

Parallel Input Serial Output (PISO)

Data bits are entered in parallel fashion.

The circuit shown below is a four bit parallel input serial output register.

Output of previous Flip Flop is connected to the input of the next one via a combinational circuit.

The binary input word B0, B1, B2, B3 is applied though the same combinational circuit.

There are two modes in which this circuit can work namely - shift mode or load mode.

Load mode

When the shift/load bar line is low (0), the AND gate 2, 4 and 6 become active they will pass B1, B2, B3 bits to the corresponding flip-flops. On the low going edge of clock, the binary input B0, B1, B2, B3 will get loaded into the corresponding flip-flops. Thus parallel loading takes place.

Shift mode

When the shift/load bar line is low (1), the AND gate 2, 4 and 6 become inactive. Hence the parallel loading of the data becomes impossible. But the AND gate 1,3 and 5 become active. Therefore the shifting of data from left to right bit by bit on application of clock pulses. Thus the parallel in serial out operation takes place.

Parallel Input Parallel Output (PIPO)

In this mode, the 4 bit binary input B0, B1, B2, B3 is applied to the data inputs D0, D1, D2, D3 respectively of the four flip-flops. As soon as a negative clock edge is applied, the input binary bits will be loaded into the flip-flops simultaneously. The loaded bits will appear simultaneously to the output side. Only clock pulse is essential to load all the bits.

Bidirectional Shift Register

If a binary number is shifted left by one position then it is equivalent to multiplying the original number by 2. Similarly if a binary number is shifted right by one position then it is equivalent to dividing the original number by 2.

Hence if we want to use the shift register to multiply and divide the given binary number, then we should be able to move the data in either left or right direction.

Such a register is called bi-directional register. A four bit bi-directional shift register is shown in fig.

There are two serial inputs namely the serial right shift data input DR, and the serial left shift data input DL along with a mode select input (M).