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Department of
Engineering Technology
LSN 12
Shift Registers
ECT 224 Digital Computer Fundamentals
ECT 224 Digital Computer [email protected]
LSN 12 – Shift Registers
• Digital circuits with data storage and data movement
functions
• Storage capacity is the total number of bits of digital
data a register can retain
– Each stage (flip-flop) can store one bit
ECT 224 Digital Computer [email protected]
LSN 12 – Shift Registers
• Shifting capability of a register allows for the
movement of data between stages of a register at a
clock edge
ECT 224 Digital Computer [email protected]
LSN 12 – Serial-In / Serial-Out Registers
• Data input serially right most bit first
• Uses a sequence of flip-flops to provide serial data
storage and shifting
– Connect in series where output of one flip-flop stage
connects to the input of the next flip-flop stage
ECT 224 Digital Computer [email protected]
LSN 12 – Serial-In / Serial-Out Registers
• Example:
– Determine outputs
for a given input
waveform if initially
reset
ECT 224 Digital Computer [email protected]
LSN 12 – Serial-In / Serial-Out Registers
• Common logic block symbol for serial I/O shift
register
– SRG = Shift ReGister
ECT 224 Digital Computer [email protected]
LSN 12 – Serial-In / Parallel-Out Registers
• The output of each stage is available simultaneously
on parallel output lines
ECT 224 Digital Computer [email protected]
LSN 12 – Serial-In / Parallel-Out Registers
• Example:
– Determine outputs for a given input waveform
ECT 224 Digital Computer [email protected]
LSN 12 – Parallel-In / Serial-Out Registers
• Data entered simultaneously to all stages on parallel
input lines
– SHIFT/LOAD input line
• When LOW, parallel inputs are loaded to the input of each stage
and stored on next active clock edge
• When HIGH, data is allowed to shift between stages and to the
output
_______
G1, G2, G3 disabled
G4, G5, G6 enabled
G1, G2, G3 enabled
G4, G5, G6 disabled
ECT 224 Digital Computer [email protected]
LSN 12 – Parallel-In / Serial-Out Registers
• Example:
– Determine output for a given input sequence and based
upon the state of the SHIFT/LOAD line_______
ECT 224 Digital Computer [email protected]
LSN 12 – Bidirectional Shift Register
• Data is able to be shifted to the right or left depending
upon gating logic
– RIGHT/LEFT input line______
ECT 224 Digital Computer [email protected]
LSN 12 – Bidirectional Shift Register
• Example:
– Assuming that
• Q0 = 1, Q1 = 1, Q2 = 0, and Q3 = 1
• Input line is LOW
ECT 224 Digital Computer [email protected]
LSN 12 – Shift Register Applications
• Time delay
– Serial in / serial out
• Delay = n ( 1/fclk)
– Example:
• Determine the time delay for a 4-bit serial to parallel shift register
between the input signal and each output signal
Number of stages
Clock period
ECT 224 Digital Computer [email protected]
LSN 12 – Shift Register Applications
• Parallel-to-serial / serial-to-parallel data conversion
– Universal Asynchronous Receiver Transmitter (UART)
• Data transmitted from microprocessor based systems in parallel
form
• Often must communicate with external devices expecting data in
serial form
– UARTs contain a parallel-to-serial converter and a serial-to-parallel
converter
ECT 224 Digital Computer [email protected]
LSN 12 – Shift Register Counters
• Shift register with feedback from serial output to
serial input
– Specified sequence of states
• Johnson Counter
– Uses the compliment of the output as the feedback term
– Produces a modulus of 2n, where n is the number of sages
Clock Pulse Q0 Q1 Q2 Q3
0 0 0 0 0
1 1 0 0 0
2 1 1 0 0
3 1 1 1 0
4 1 1 1 1
5 0 1 1 1
6 0 0 1 1
7 0 0 0 1
ECT 224 Digital Computer [email protected]
LSN 12 – Shift Registers
– Ring Counter
• The shift register output is used as the feedback term
• Load a sequence of bits into the counter and the sequence is shifted
one place around the “ring” at each clock edge
Clock Pulse Q0 Q1 Q2 Q3 Q4
0 1 0 0 0 0
1 0 1 0 0 0
2 0 0 1 0 0
3 0 0 0 1 0
4 0 0 0 0 1
ECT 224 Digital Computer [email protected]
LSN 12 – Homework
• Reading
– Chapter 9.1 – 9.7
• Assignment – HW13
– Chapter 9, problems 5, 10, 13, 21