Chapter 13 Shift Registers William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River,

Chapter 13

Shift Registers

William KleitzDigital Electronics with VHDL, Quartus® II Version

Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Shift Register Basics

• Receives 4 bits of parallel data and shifts them to to right

• Timing provided by the input clock

• Shift right by one position each clock pulse

• See Figure 13-1

• Parallel format or Serial format

• Shift Register operations - See Figure 13-2William KleitzDigital Electronics with VHDL, Quartus® II Version



Figure 13-1

Figure 13-2




Parallel-to-Serial Conversion

• 4-bit parallel-in, serial-out shift register

• See Figure 13-3




Figure 13-3




Recirculating Register

• Rightmost data bit goes back to the input

• See Figure 13-3




Serial-to-Parallel Conversion

• Serial-in, Parallel-out

• See Figure 13-4

• Inverter required at Ds

• Strobe line used to enable-then-disable the clock to stop the process




Figure 13-4




Ring Shift Counter and Johnson Shift Counter

• Used to control digital sequences

• 4-bit ring shift counter– See Figure 13-5

• Johnson Shift counter– See Figure 13-6




Figure 13-5

Figure 13-6




VHDL Description of Shift Registers

• Design flexibility– shift left or shift right– synchronous or asynchronous parallel loading– strobing the clock– recirculating data bits




VHDL Description of Shift Registers

• Serial-in shift right shift register– see figure 13-7 and 13-8

• Parallel load shift right shift register– see figure 13-9 and 13-10




Figure 13-7

Figure 13-8




Figure 13-9

Figure 13-10




Shift Register ICs

• 74164 8-bit Serial-In, Parallel-Out

• 2 serial input lines

• Positive edge-triggered clock

• Active-LOW Master Reset

• See Figure 13-11– logic symbol– logic diagram




Figure 13-11




Shift Register ICs

• 74165 8-bit Serial or Parallel-In, Serial-Out

• Active-LOW parallel load

• Active-LOW clock enable

• Positive edge-triggered clock

• Serial output and its complement

• See Figure 13-13




Figure 13-13




Shift Register ICs

• 74194 4-bit bidirectional universal

• Mode Control inputs– See Table 13-2

• Make recirculating by connecting Q3 back to DSR








Figure 13-14




Shift Register ICs

• Three-State Outputs– HIGH, LOW or float (high-impedance state)– used to connect output of more than one

register to the same points– 74395A 4-bit shift register with tri-state outputs– See Figure 13-18

• logic symbol

• logic circuit




Figure 13-18




System Design Applications for Shift Registers

• Traffic light controller– See Example 13-4

• 16-bit serial-to-parallel converter– See Example 13-5

• Parallel-to-serial conversion– See Example 13-6

• Interface to an 8-bit serial printer– See Example 13-7
















Driving a Stepper Motor with a Shift Register

• Stepper motor makes rotation in steps

• Driven by sequential digital signals

• Made of stator with pole pairs and rotor with ferromagnetic pairs


• Drive Circuitry - See Figure 13-27




Figure 13-24

Figure 13-27




Three-State Buffers, Latches and Transceivers

• Three State Buffers– when output devices must share same data bus– provides isolation– active-LOW Output Enable

• Octal Latches/Flip-Flops– eight data lines simultaneously– See Figure 13-30




Figure 13-30




Three-State Buffers, Latches and Transceivers

• Transceivers– transmitter/receiver– bidirectional– for both input to and output from a

microprocessor– See Figure 13-32




Figure 13-32




Using the LPM Shift Register and 74194 Macrofunction

• General purpose shift register in the library of parameterized modules: LPM_SHIFTREG– port/parameters options

• LPM_WIDTH

• serial or parallel I/O

• asynch or synch loading

• clock enable

– example of serial in, shift right, with synch parallel loading

– see figure 13-34 and 13-35William KleitzDigital Electronics with VHDL, Quartus® II Version



Figure 13-34

Figure 13-35




Using the LPM Shift Register and 74194 Macrofunction

• 7400 series are provided as macrofunctions

• The 74194 as a universal shift register– see figure 13-36 and 13-37




Figure 13-36

Figure 13-37




Summary

• Shift registers are used for serial-to-parallel and parallel-to-serial conversions.

• One common form of digital communication is for a sending computer to convert its data from parallel to serial and then transmit over a telephone line to a receiving computer, which converts back from serial to parallel.




Summary• Simple shift registers can be constructed by

connecting the Q-outputs on one J-K flip-flop into the J-K inputs of the next flip-flop. Several flip-flops can be cascaded together this way, driven by a common clock, to form multibit shift registers.

• The ring and Johnson shift counters are two specialized shift registers used to create sequential control waveforms.




Summary

• Several multibit shift register ICs are available for the designer to choose from. They generally have four or eight internal flip-flops and are designed to shift either left or right and perform either serial-to-parallel or parallel-to-serial conversions.

• The 74194 is called a universal 4-bit shift register, because it can shift in either direction and can receive and convert to either format.




Summary

• Three-state outputs are used on ICs that must have their outputs go to a common point. They are capable of the normal HIGH/LOW levels but can also output a float (or high-impedance) state.

• The rotation of a stepper motor is made by taking small angular steps. This is controlled by sequential digital strings often generated by a recirculating shift register.




Summary

• Three-state buffers, latches and transceivers are an integral part of microprocessor interface circuitry. They allow the microprocessor system to have external control of 8-bit groups of data. The buffer can be used to allow multiple inputs devices to feed a common point or to provide high output current to a connected load.




Summary

• The latch can be used to remember momentary data from the microprocessor that needs to be held for other devices in the system. The transceiver provides bidirectional (input or output) control of interface circuitry.




Documents

Chapter 13 Shift Registers William Kleitz Digital Electronics with VHDL, Quartus® II Version Copyright ©2006 by Pearson Education, Inc. Upper Saddle River,