Shift Registers pp

2 Shift Registers Capability to shift bits ♦ In one or both directions Why? ♦ Part of standard CPU instruction set ♦ Cheap multiplication ♦ Serial communications Just a chain of flip-flops

3 Simple 4-Bit Shift Register In the simplest case, the shift register is simply a set of D flip-flops connected in a row like this: Shift Registers move data laterally within the register toward its MSB (Most Significant Bit), or LSB (Least Significant Bit) position

4 Simple 4-Bit Shift Register (continued) Data input, In, is called a serial input or the shift right input. Data output, Out, is often called the serial output.

5 Simple 4-Bit Shift Register (continued) Clocked in common The vector (A, B, C, Out) is called the parallel output.

6 Symbol

7 Shift Register with Parallel Load Can provide parallel outputs from flip-flops And also parallel inputs

8 Shift Register with Parallel Load (continued) Shift Parallel load “no change” Schematic next

9 Fig Shift Register with Parallel Load Function Table

10 Why is this useful? Basis for serial communications ♦ Keyboard ♦ Serial port Now mainly for modems

11 Bidirectional Shift Register Shift either way Now we have following possible inputs ♦ Parallel load ♦ Shift from left ♦ Shift from right ♦ Also “no change” Schematic next

12 Schematic

 Shift registers can also be designed to shift more than a single bit position right or left  Shift register can be designed to shift a variable number of bit positions specified by a variable called a shift amount. Shift Registers with Additional Functions