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  1. 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.


  3. 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

  4. 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.


  6. 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. 

  7. Serial In - Serial Out Shift Register Circuit Diagram:

  8. Serial In - Serial Out Shift Register The 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

  9. 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.

  10. Serial In - Serial Out Shift Register

  11. 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.

  12. Serial In - Parallel Out Shift Registers

  13. 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.

  14. Parallel In - Serial Out Shift Registers

  15. 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.

  16. Parallel In - Parallel Out Shift Registers

  17. 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.

  18. Bidirectional Shift Register

  19. Bidirectional shift register