1. Registers
CPE 49
RMUTI
KOTAT

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
Clocked in common
Just serial in and serial out
Not quite a FIFO

4. Serial In - Serial Out Shift Registers

5. Serial In - Parallel Out Shift Registers

6. Parallel In - Serial Out Shift Registers

7. Parallel In - Parallel Out Shift Registers

8. Bidirectional Shift Registers

9. Shift Register Counters
Ring Counters

10. Shift Register Counters
Johnson Counters

11. 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.

12. 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.

13. Symbol
we could gate the clock

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

15. Schematic

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

17. 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

18. Schematic

19. 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];
CLK or posedge RESET)
begin
if (RESET)
Q <= 4'b0000;
else
Q <= {Q[2:0], SI};
end
endmodule

20. 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

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

22. Table Showing Shift
Hopefully we’ve figured it out

23. 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

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

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

26. Circuit and State Diagram

27. 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

28. References