REGISTER A Register is a group of binary storage cells suitable for holding binary information.A group of flip-flops constitutes a register, since each flip-flop is a binary cell capable of storing one bit of information.An n-bit register has a group of n flip-flops and it capable of storing any binary information containing n bits.A Register consists of a group of flip-flops and gates that effect their transition.

The flip-flops hold binary information and The gates control when and how new information is transferred into the register.

REGISTER

REGISTER

A Register constructed with four D-type flip-flops and a common clock pulse output.

The clock pulse, CP, enables all flip-flops so that the informations presently available at the four inputs can be transferred into the 4-bit register.

There are two types of registers: parallel and serial registers. They differ in the manner in which the binary data is loaded and retrieved from them

SHIFT REGISTER

A Register capable of shifting its binary information either to the right or to the left is called a “Shift Register”.

It consists of a chain of flip-flops connected in cascade, with the output of one flip-flop connected to the input of the next flip-flop.

SHIFT REGISTER

SHIFT REGISTER

The basic types of shift registers are as follows,

Serial In - Serial Out, Serial In - Parallel Out, Parallel In - Serial Out, Parallel In - Parallel Out, and bidirectional shift registers. 

Serial In - Serial Out Shift RegisterCircuit Diagram:

Serial In - Serial Out Shift RegisterThe difference between serial transfer and

parallel transfer is: In the parallel mode information is

available from all bits of register and all bits can be transferred simultaneously during one clock pulse.

In the serial mode, the registers have a single serial input and a single serial output.

The information is transferred one bit at a time while registers are shifted in the same direction

Serial In - Serial Out Shift Register

To prevent the loss of information stored in the source register, the serial output of A is connected to its serial input.

The shift control input determines when and how many times the register are shift.

Here each register has four bits and each rising edge of the pulse cause a one bit shift in each register.

For a fixed time of four clock pulses the content of A is transferred into B.

Serial In - Serial Out Shift Register

Serial In - Parallel Out Shift Registers For this kind of register, data bits are

entered serially in the same manner as discussed in the last section.  

The difference is the way in which the data bits are taken out of the register.  

Once the data are stored, each bit appears on its respective output line, and all bits are available simultaneously.

Serial In - Parallel Out Shift Registers

Parallel In - Serial Out Shift Registers The circuit uses D flip-flops and NAND

gates for entering data (ie, writing) to the register.

D0, D1, D2 and D3 are the parallel inputs, where D0 is the most significant bit and D3 is the least significant bit.  

To write data in, the mode control line is taken to LOW and the data is clocked in.  

The data can be shifted when the mode control line is HIGH as SHIFT is active high.

Parallel In - Serial Out Shift Registers

Parallel In - Parallel Out Shift Registers For parallel in - parallel out shift

registers, all data bits appear on the parallel outputs immediately following the simultaneous entry of the data bits.  

The D's are the parallel inputs and the Q's are the parallel outputs.  

Once the register is clocked, all the data at the D inputs appear at the corresponding Q outputs simultaneously.

Parallel In - Parallel Out Shift Registers

Bidirectional shift register

A bidirectional, or reversible, shift register is one in which the data can be shift either left or right.  

One that can shift in only one direction is called a “unidirectional shift register”

If the Register has both shift and parallel-load capabilities, it is called a “shift register with parallel load”

A four-bit bidirectional shift register using D flip-flops is shown in next slide.

Bidirectional Shift Register

Bidirectional shift register

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S1S0 Action00 Nothing01 Shift down10 Shift up11 Parallel load