Shift Register Basics

• A register acts as a temporary storage device for a group of data bits.
• A shift register is used to move data to the left or to the right by one bit for each input clock pulse.
• Parallel load means to load all flip-flops of a register at one time.
• Serial load means to load the flip-flop of a register one bit at a time.
• The number of clock pulses required to shift all bits of a register completely in or completely out of the register is equal to the number of flip-flops in the register.
• There are four basic register configurations:
1. Parallel-in, Parallel-out.
2. Serial-in, Serial-out (both left shift and right shift).
3. Parallel-in, Serial-out (right shift).
4. Serial-in, Parallel-out (right shift).

• Many computers operate on parallel data; but, this data must be converted to serial format to be sent over phone lines.
• ICs called UARTs are used to interface microprocessors and parallel data to communications links that use serial data.
• This parallel-to-serial conversion and serial-to-parallel conversion can be performed by shift registers.
• Shift registers are available in IC form or can be constructed from discrete flip-flops as is shown here with a five-bit serial-in serial-out register.
• Each clock pulse will move an input bit to the next flip-flop. For example, a 1 is shown as it moves across.

 

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