Registers CPE 49 RMUTI KOTAT

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

Simple 4-Bit Shift Register Clocked in common Just serial in and serial out Not quite a FIFO

Serial In - Serial Out Shift Registers

Serial In - Parallel Out Shift Registers

Parallel In - Serial Out Shift Registers

Parallel In - Parallel Out Shift Registers

Bidirectional Shift Registers

Shift Register Counters Ring Counters

Shift Register Counters Johnson Counters

Applications To produce time delay To simplify combinational logic The serial in -serial out shift register can be used as a time delay device.  The amount of delay can be controlled by: - the number of stages in the register - the clock frequency To simplify combinational logic The ring counter technique can be effectively utilized to implement synchronous sequential circuits.  A major problem in the realization of sequential circuits is the assignment of binary codes to the internal states of the circuit in order to reduce the complexity of circuits required.  By assigning one flip-flop to one internal state, it is possible to simplify the combinational logic required to realize the complete sequential circuit.  When the circuit is in a particular state, the flip-flop corresponding to that state is set to HIGH and all other flip-flops remain LOW.

Applications To convert serial data to parallel data A computer or microprocessor-based system commonly requires incoming data to be in parallel format.  But frequently, these systems must communicate with external devices that send or receive serial data.  So, serial-to-parallel conversion is required.  As shown in the previous sections, a serial in - parallel out register can achieve this.

Symbol we could gate the clock

Parallel Load Can provide parallel outputs from flip-flops And also parallel inputs

Schematic

Why is this useful? Basis for serial communications Keyboard Serial port Initially to connect to terminals Now mainly for modem

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

Schematic

Verilog for Shift Register module srg_4_r_v (CLK, RESET, SI, Q,SO); input CLK, RESET, SI; output [3:0] Q; output SO; reg [3:0] Q; assign SO = Q[3]; always@(posedge CLK or posedge RESET) begin if (RESET) Q <= 4'b0000; else Q <= {Q[2:0], SI}; end endmodule

Serial Transfer Parallel transfer – over as many wires as word (for example) Serial transfer – over a single wire Trade time for wires Takes n times longer

Example Why do this? Maybe these are far apart Could shift data in What’s on wire at each clock? Clocked 4 times

Table Showing Shift Hopefully we’ve figured it out

Then shift through adder into A. Added to 0 if A is empty. Serial Addition Initially reset all registers Register A accumulates Adds one bit at a time At same time, new value going into B Stores carry one clock Shift value in serially

Hardware Comparison Serial vs. parallel adder One full adder vs. n adders Serial takes n units of time, parallel only one

Arbitrary Count One more type of counter is useful Count an arbitrary sequence Maybe you need a sequence of states

Circuit and State Diagram

Shift Registers N-bit register with the provision for shifting its stored data by a bit position each tick of the clock. Serial input – specifies a new bit to shifted into the register. Serial output – specifies the bits being shifted out of the register

References http://www.eelab.usyd.edu.au/digital_tutorial/part2/hpage.html http://www.allaboutcircuits.com/vol_4/index.html