ECE U322 Digital Logic Design

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ECE U322Digital Logic Design
Oct. 19, 2005

• Lecture 17
• Registers
• Reading Marcovitz 6.2
• Homework 4 due Thursday, Oct. 20.

lect17.ppt
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Demultiplexer

• The demultiplexer is actually identical to a
  decoder with enable, as illustrated below

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Combinational Logic

• So far, everything we have done is combinational
  logic
• Inputs change gt outputs change
• I can determine what the output is by knowing the
  current inputs
• I dont need any other information

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Sequential Logic

• Sequential Logic
• Combinational Logic Memory
• Why do we need memory ?
• Combinational logic
• can add two numbers. But
• No way of adding two numbers, then adding a third
  (a sequential operation)
• No way of remembering or storing information
  after inputs have been removed.

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Example Alarm Clock
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Sequential Circuits Get memory through
feedback Outputs of circuit feed back to
inputs
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Latches

• Latches are the simplest memory elements
• A latch stores one bit of data
• Latches are built from logic gates
• Get memory with feedback
• connect outputs back to inputs
• Flip-flops are made of latches.

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D Flip-Flop
preset
S
clear
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D Flip-Flop Timing
D
C
QFF
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Registers

• Perform data-processing tasks.
• Comprised of
• Flip-flops
• To hold data.
• Gates
• To determine the new or transformed data to be
  transferred into the flip-flops.
• An n-bit register stores n bits.

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4-bit Register
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• Load
• transfer of new information into a register.

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4-bit Register Symbol
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Register with Parallel Load
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Shift Registers

• Registers capable of shifting stored bits
  laterally in one or both directions.

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4-bit Shift Register
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Serial Mode

• When information in the system is transferred or
  manipulated one bit at a time.

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Serial Transfer

• Information is transferred one bit at a time by
  shifting the bits out of one register and into a
  second register.

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Serial Transfer
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Example of serial transfer
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Difference Between Serial and Parallel

• Parallel
• information is available from all bits of a
  register, and all bits can be transferred
  simultaneously during one clock pulse.
• Serial
• the registers have a single serial input and a
  single serial output, and information is
  transferred one bit at a time.

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Space and Time Trade-off

• Operate in parallel, because of the faster speed
  attainable.
• Serial operations are slower, but have the
  advantage of requiring less hardware.

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Serial Addition
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Recall Parallel Adder
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Comparison between serial and parallel adder
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Shift Register with Parallel Load
Symbol
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• Unidirectional shift register
• Capable of shifting in only one direction.
• Bi-directional shift register
• Can shift in both directions.

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Bi-directional Shift Register
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Function Table
Symbol
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Midterm1 Statistics

• Average 79
• Median 84
• Standard Deviation 21
• Max 100