Shift Register
Section 6.1-6.2
Register
• A register is a group of flip-flops,
each one of which is capable of
storing one bit of information.
• Issues of the circuit to the right.
– You do not have an option hold
the output when you don’t want
to outputs updated.
4 D flip-flops=4 bits of storage=4-bit register
4-bit Register with Parallel Load
Control
Load=“1”→Update
“1”
“0”
I0 is fed to DFF when
Load is a 1.
“1”
“0” “I ”
0
“I0”
Load=“0”→Hold!
A is fed to DFF when
“0”
“1”
0
Load is a 0. So the output
is holding!
“0”
“A0” “A ”
0
“0”
We will revisit this idea
when we study the universal
shift register.
Four-Bit Serial Shift Register
1
2
3
4
Q of DFF1 gets SI after the first rising edge of the CLK
Q of DFF2 gets SI after the second rising edge of the CLK
Q of DFF3 gets SI after the third rising edge of the CLK
Q of DFF4 gets SI after the fourth rising edge of the CLK
Linear Feedback Shift Register
1
1
0
1
Exclusive OR
Content of Four-Bit Shift Register
1
1
0
1
1
1
1
0
1
1
1
1
0
1
1
1
0
0
1
1
0
0
0
1
1
0
0
0
0
1
0
0
0
0
1
0
1
0
0
1
1
1
0
0
0
1
1
0
1
0
1
1
0
1
0
1
1
0
1
0
Block Diagram of a Universal
Shift Register
This is called the universal shift register because it has both shifts and
parallel load capabilities.
Functionality of the Universal
Shift Register
Clear: to clear the register to 0.
CLK: to synchronize the operations.
{S1,S0} for mode control.
A_par: register output
I_par: register input
MSB_in and LSB_in: serial inputs
Detail Implementation
Four-to-one-line Mux
Four-to-one-line Mux
0
0
I2
1
1
0
1
0
0
0
I2
I2
0
Mode Control
S0=0, S1=0 [No Change Mode]
S0=0, S1=0
S0=1, S1=0 [Shift Right Mode]
S1=0 , S0=1
S0=0, S1=1 [Shift Left Mode]
S1=1 , S0=0
S0=1, S1=1 [Parallel Load Mode]
S1=1 , S0=1
Breadboard Implementation
Universal shift regsiter
Random Number Generator
Waveform
Random
A3
A2
A1
A0
CLK
4-Bit Universal Shift Register
Behavioral Vs. Structural
Description
• Behavioral Description
– Behavior model of a shift register
• Describe the operation of the register without
a preconceived structure.
– Random number generator
• Binary values of msb_in, lsb_in, i_par
• Structural Description
– Models the circuits in terms of a collection
of components such as gates, flip-flops…
Behavioral Model of Shift
Regsiter
a_par[3]
a_par[2]
a_par[1]
a_par[0]
Test Bench
Test all input combinations by flipping {S1, S0}
1. Generate random number
With matlab
2. Read random number
at the neg edge of the clock
Read numbers to i_par[3:0],msb_in, lsb_in
at the negedge of t_clock
[s1,s0=[1,1], Load
i_par=0111
a_par=0111
[s1,s0]=[0,0], No Change
i_par=0111
a_par=0011
[s1,s0]=[1,0], Shift Left
1101
1011
LSB_in
[s1,s0]=[0,1], Shift Right
Synthesized Schematic
Structural Modeling of a 4-Bit
Universal Shift Register
Q
clr
clk
select
i3 i1
i2 i0
Waveform
Load
No Change
Shift Right
Shift Left
4-bit Universal Shift Register
Verilog Code of Each Stage
In-Class Exercise
Load=“1”→Update
“1”
“0”
I0 is fed to DFF when
Load is a 1.
“1”
“0” “I ”
0
“I0”
Load=“0”→Hold!
A is fed to DFF when
“0”
“1”
0
Load is a 0. So the output
is holding!
“0”
“A0” “A ”
0
“0”
We will revisit this idea
when we study the universal
shift register.
S0=0, S1=0 [No Change Mode]
S0=0, S1=0
S0=1, S1=0 [Shift Right Mode]
S1=0 , S0=1
S0=0, S1=1 [Shift Left Mode]
S1=1 , S0=0
S0=1, S1=1 [Parallel Load Mode]
S1=1 , S0=1
If time permits
Serial Transfer Using Shift Register
Information in A is made to
circulate by connecting SO to
SI.
Parallel Transfer Versus Serial
Transfer
(Serial Transfer)
Parallel Transfer
Transfer all the bit in one clock cycle.
Require combinatorial circuits.
Take multiple clock cycles
to transfer data.
Assume n=4, each shift
Register has 4 DFF.
Augend, Addend & Sum
1011
+1001
______
10100
Augend
Addend
Sum
Serial Adder
Assuming a shift-right register, the left most
position becomes available
for storage after the second rising edge of
the clock.
1
(Augend)
1
(Addend)
0
1
Feed “1” to z
at the next rising edge
of the CLK
Note that
The sum can be
stored in a third
register.
But if you want to save shift register, you can store it in A since more and more
slots in SRA become available.
Serial Adder At the end of T4
S2S1S0A3
S3
Co
D2D1D0B3
A3A2A1A0
B3B2B1B0
________________
CoS3S2S1S0
Allowing the Serial Adder to
Accumulate
T2T1T0S3
T3
Ro
X2X1X0D3
Co
S3 S2 S1S0
D3D2D1D0
________________
Ro T3 T2 T1 T0
Accumulate with a Shift Register
A, B and D, each represents a 4 bit sequence.
We want to perform A+B+D
Store A in shift register A.
Store B in shift register B.
Allow the CLK to go on for a couple of cycles.
Store the sum bits of A+B in Shift A and allow D
to enter shift register B.
• Allow more cycles of CLK.
• Add D to A+B, and allow A+B+C to enter shift
register A.
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