Shift register counters are shift registers with feedback that exhibit ?special sequences. Examples are the Johnson counter and the ring ?counter. The Johnson counter has 2n states i
Shift register counters are shift registers with feedback that exhibit special sequences. Examples are the Johnson counter and the ring counter. The Johnson counter has 2n states in its sequence, where n is the number of stages. The ring counter has n states in its sequence.
answer the following:
- Describe how a bidirectional shift register operates
- Discuss the operation of a basic shift-register-controlled keyboard encoder
1
A bidirectional shift register is a type of shift register that allows the data to be shifted either left or right depending upon the mode selected. Here's a brief description of how it works:
Data Input: The process begins by loading data into the register. The data is represented in binary form, where each bit is loaded into a flip-flop (the basic unit of a shift register).
Direction of Shift: The direction in which the data is shifted (either left or right) depends on the control input. If the control input (often referred to as the shift-right/shift-left control) is set to "right", the data will be moved to the right. If the control is set to "left", the data will be moved to the left.
Shifting Operation: When a clock pulse is applied, the data is shifted in the direction specified by the control input. The bit on the far side (left for a right shift, right for a left shift) is ejected from the register and a new bit enters from the opposite side.
A shift-register-controlled keyboard encoder operates by assigning a unique binary code to each key on a keyboard. When a key is pressed, its associated binary code is loaded into the shift register. The encoder will then sequentially shift this code bit by bit onto a data line whenever a clock pulse is received. This process converts the parallel input from the keyboard into a serial output that can be read by the computer.
The encoder continues to shift out the binary code for the pressed key until it receives a reset signal, which typically occurs when the key is released. If another key is pressed before the first key is released, the encoder loads the new key's binary code into the shift register and begins to shift it out, effectively giving the keyboard "type-ahead" capability.
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EET 230– Digital Systems II
Registers
2
Outline of the lecture
Basic Shift Register Functions
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
Objective of the Lecture
After successful completion of the lecture students
will be able to:
Identify the basic forms of data movement in shift registers
Explain how SISO, SIPO, PIPO and PISO registers
operate
Explain how a bi-directional shift register operates
Determine the sequence of Johnson counter
Construct ring counter from shift registers
Enumerate practical applications of shift registers
3
4
Registers
A register is a collection of flip-flops that act as an
extension of a flip-flop which can store multiple bits
It has two basic functions: Data Storage and Data
Movement
A shift register provides the data movement function
where it is capable of shifting their binary contents in one
or both directions (shifting data to the left or right)
It “shifts” its output once every clock cycle
5
A Basic Register
Basic registers are easy to build. We can store multiple bits just by putting a bunch of flip-flops together!
A 4-bit register with its internal implementation is as shown below This register uses D flip-flops
All the flip-flops share a common CLK and CLR signal
6
Shift Registers
A shift register "shifts" its output once every clock cycle
SI is an input that supplies a new bit to shift "into" the register
For example, if on some positive clock edge we have:
SI = 1 Q0-Q3 = 0110 then the next state will be:
Q0-Q3 = 1011
The current Q3 (0 in this example) will be lost on the next cycle
Q0(t+1) = SI Q1(t+1) = Q0(t) Q2(t+1) = Q1(t) Q3(t+1) = Q2(t)
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Basic Shift Register Operation
Some basic data movements are illustrated here:
Data in Data in
Data in
Data in
Data in
Data out Data out
Data out Data out
Serial in/shift right/serial out Serial in/shift left/serial out Parallel in/serial out
Parallel in/parallel outSerial in/parallel out Rotate right Rotate left
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Data Movement
Block diagrams for shift registers with various input/output options:
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Serial Transfer vs. Parallel Transfer
Serial transfer
Information is transferred one bit at a time
Shifts the bits out of the source register into the destination register
Parallel transfer:
All the bits of the register are transferred at the same time
For example:
Older printers use parallel data transfer
USB devices use serial data transfer
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Parallel Data Transfer
Parallel data transfer moves data from one register to another at one time
clock
Reg. A Reg. B
When clock occurs, all bits of A are copied to B
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Serial Data Transfer
One application of shift registers is converting between
"serial data" and "parallel data" Serial transfer moves data bits from A to B one bit per clock Rx and Tx have single wire between the two For ‘n’ bit registers, it takes ‘n’ clocks for data move
Reg. A
1 bit signal
Usual implementation is with a shift register
Reg. B
clock
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Serial Data Transfer (Cont.)
Typical serial transfer is a multi-step process
Load transmit shift register with data to send
Shift data bit by bit from transmit to receive shift register
Transfer received data to other registers
The transmit shift register must have parallel load
This is known as parallel to serial shift register
The receive shift register must have parallel outputs
This is known as serial to parallel shift register
Other control/timing signals usually needed
Serial In Serial Out Shift Register
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Serial In/Serial Out Shift Registers
5-bit version
Serial In/Parallel Out Shift Registers
4-bit serial in/parallel out
Waveforms
Serial In/Parallel Out Shift Registers
4-bit serial in/parallel out
Serial In/Parallel Out Shift Registers
8-bit serial in/parallel out
Serial In/Parallel Out Shift Registers
8-bit serial in/parallel out
Parallel In/Serial Out Shift Registers
4-bit parallel in/serial out
Waveforms
Parallel In/Serial Out Shift Registers
4-bit parallel in/serial out
Parallel In/Serial Out Shift Registers
8-bit version
Waveforms
Parallel In/Serial Out Shift Registers
8-bit version
Parallel In/Parallel Out Shift Registers
4-bit version
Waveforms
Parallel In/Parallel Out Shift Registers
4-bit version
Bidirectional Shift Register
4-bit serial in/serial out
Bidirectional Shift Register
4-bit universal
27
Summary
Basic Shift Register Functions
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
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