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Shift Registers
1. EET 3350 Digital Systems Design
Textbook: John Wakerly
Chapter 8: 8.5
Shift Registers
1
2. Definition
• A register is a digital circuit with two basic
functions: Data Storage and Data Movement
– A shift register provides the data movement
function
– A shift register “shifts” its output once every clock
cycle
• A shift register is a group of flip-flops set up in
a linear fashion with their inputs and outputs
connected together in such a way that the data
is shifted from one device to another when the
circuit is active
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3. Shift Register Applications
• converting between • some counter
serial data and applications
parallel data – ring counter
• temporary storage in – Johnson counter
a processor – Linear Feedback Shift
– scratch-pad memories Register (LFSR) counters
• some arithmetic • time delay devices
operations • more …
– multiply, divide
• communications
– UART
3
4. Shift Register Characteristics
• Types
– Serial-in, Serial-out
– Serial-in, Parallel-out
– Parallel-in, Serial-out
– Parallel-in, Parallel-out
– Universal n-bit shift
register
• Direction
– Left shift
– Right shift
– Rotate (right or left)
– Bidirectional
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5. Data Movement
• The bits in a shift register can move in any of the
following manners
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6. Data Movement
• Block diagrams for shift registers with various
input/output options:
n-bit shift n-bit shift
register register
n-bit shift n-bit shift
register register
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7. Serial-In Serial-Out
• Data bits come in one at a
time and leave one at a time n-bit shift
register
• One Flip-Flop for each bit to
be handled
• Movement can be left or
right, but is usually only in a
single direction in a given
register
• Asynchronous preset and
clear inputs are used to set
initial values
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8. Serial-In Serial-Out
• The logic circuit diagram below shows a
generalized serial-in serial-out shift register
– SR Flip-Flops are shown
– Connected to behave as D Flip-Flops
– Input values moved to outputs of each Flip-Flop
with the clock (shift) pulse
N 1 0
N-Bit Shift Register
8
9. Shift Registers
• The simplest shift register is one that uses only Flip-Flops
• The output of a given Flip-Flop is connected to the D input of the
Flip-Flop at its right.
• Each clock pulse shifts the contents of the register one bit
position to the right.
• The Serial input (SI) determines what goes into the leftmost Flip-
Flop during the shift. The Serial output (SO) is taken from the
output of the rightmost Flip-Flop.
Q Q Q Q
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10. Serial-In Serial-Out
• A simple way of looking
at the serial shifting
operation, with a focus on 1
the data bits, is illustrated
at right 2
• The 4-bit data word
“1011” is to be shifted into 3
a 4-bit shift register
• One shift per clock pulse 4
• Data is shown entering at
left and shifting right
5
10
11. Serial-In Serial-Out
• The diagram at right
shows the 4-bit
sequence “1010”
being loaded into the
4-bit serial-in serial-
out shift register
• Each bit moves one
position to the right
each time the clock’s
leading edge occurs
• Four clock pulses
loads the register
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12. Serial-In Serial-Out
• This diagram shows
the 4-bit sequence
“1010” as it is
unloaded from the 4-
bit serial-in serial-out
shift register
• Each bit moves one
position to the right
each time the clock’s
leading edge occurs
• Four clock pulses
unloads the register
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13. Serial-In Serial-Out
• Serial-in, serial-out
shift registers are
often used for data
communications
– such as RS-232
– modem transmission
and reception
– Ethernet links
– SONET
– etc.
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14. Serial-to-Parallel Conversion
• We often need to convert
from serial to parallel n-bit shift
register
– e.g., after receiving a series
transmission
• The diagrams at the right
illustrate a 4-bit serial-in
parallel-out shift register
• Note that we could also use
the Q of the right-most Flip-
Flop as a serial-out output
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15. Serial-to-Parallel Conversion
• We would use a
serial-in parallel-out
shift register of
arbitrary length N to
convert an N-bit word
from serial to parallel
• It would require N
clock pulses to LOAD
and one clock pulse
to UNLOAD
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16. Serial-to-Parallel Conversion
• These two shift
registers are used
to convert serial
data to parallel data
• The upper shift
register would
“grab” the data
once it was shifted
into the lower
register
16
17. Parallel-to-Serial Conversion
• We use a Parallel-in Serial-out
Shift Register n-bit shift
register
• The DATA is applied in parallel
form to the parallel input pins PA
to PD of the register
• It is then read out sequentially
from the register one bit at a time
from PA to PD on each clock cycle
in a serial format
• One clock pulse to load
• Four pulses to unload
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19. Parallel-In Parallel-Out
• Parallel-in Parallel-out Shift
Registers can serve as a
temporary storage device or
as a time delay device
• The DATA is presented in a
parallel format to the parallel
input pins PA to PD and then
shifted to the corresponding
output pins QA to QD when
the registers are clocked
• One clock pulse to load
• One pulse to unload
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20. Universal Shift Register
• Universal shift register
• Can do any combination of
parallel and serial n-bit shift
register
input/output operations
• Requires additional inputs to
specify desired function
• Uses a Mux-like input gating
L/S L/S
A 0
0 F A
B 1 B
1
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22. Universal Shift Register
• Parallel shift register (can serve as converting
parallel-in to serial-out shifter):
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23. MSI Shift Registers
• 74LS164 is an 8-Bit Serial-
In Parallel-Out Shift
Register
• Typical Shift Frequency of
35 MHz
• Asynchronous Master
Reset
• Gated Serial Data Input
• Fully Synchronous Data
Transfers
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24. MSI Shift Registers
• 74LS164 logic diagram
A LOW level on the Master Reset (MR) input overrides all other
inputs and clears the register asynchronously, forcing all Q outputs
LOW.
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26. MSI Shift Registers
• The 74LS164 is an edge-
triggered 8-bit shift register with
serial data entry and an output
from each of the eight stages.
• Data is entered serially through
one of two inputs (A or B);
– either of these inputs can be used
as an active HIGH Enable for data
entry through the other input
– an unused input must be tied HIGH,
or both inputs connected together
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27. MSI Shift Registers
• Each LOW-to-HIGH transition on the Clock (CP) input
shifts data one place to the right
• This also outputs at Q0 the logical AND of the two data
inputs (A•B) that existed before the rising clock edge.
27
29. MSI Shift Registers
• 74LS166 is an 8-Bit Shift
Register
• Parallel-in or serial-in
– shift/load input establishes
the parallel-in or serial-in
mode
• Serial-out
• Synchronous Load
– Serial data flow is inhibited
during parallel loading
• Direct Overriding Clear
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