Practical Considerations for Digital Design

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Chapter 11

• Practical Considerations for Digital Design

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Objectives

• You should be able to
• Describe the causes and effects of a race
  condition on synchronous flip-flop operation.
• Use manufacturers data sheets to determine IC
  operating specifications such as setup time, hold
  time, propagation delay, and input/output voltage
  and current specifications.

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Objectives

• (Continued)
• Perform worst-case analysis on the time-dependent
  operations of flip-flops and sequential
  circuitry.
• Design a series RC circuit to provide an
  automatic power-up reset function

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Objectives

• (Continued)
• Describe the wave-shaping capability and
  operating characteristics of Schmitt trigger ICs.
• Describe the problems caused by switch bounce and
  how to eliminate its effects.
• Calculate the optimum size for a pull-up resistor.

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Flip-Flop Time Parameters

• Race condition inputs are changing at the same
  time as the active clock edge

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Flip-Flop Time Parameters

• Data Manual provides ac waveforms to illustrate
  measuring points
• Setup time how far back the FF looks to
  determine input levels

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Flip-Flop Time Parameters

• Hold time how long the input level must be held
  beyond the active clock edge

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Flip-Flop Time Parameters

• Propagation delay - delay from input to output

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Flip-Flop Time Parameters

• Other information in Manufacturers Data Manual
• Maximum frequency (fmax)
• Clock pulse width tw (L) and tw (H)
• Set or reset pulse width tw (L)
• Metastable state
• Output voltage at an invalid level
• Delay gate ICs provide exact delays

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Discussion Point

• Locate the following items on the sample data
  sheet shown in Figure 11-11
• Pin configuration
• Function table
• Propagation delay
• Setup and hold times

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Automatic Reset

• RC circuit to reset at power-up

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Schmitt Trigger ICs

• Transforms slowly changing waveforms to sharply
  defined output signals

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Schmitt Trigger ICs

• Positive feedback is used to speed up level
  transitions
• Hysteresis
• Switching thresholds of positive and
  negative-going signals are different
• Useful to ignore small amounts of jitter

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Schmitt Trigger ICs

• Transfer Function

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Switch Debouncing

• Switch bounce occurs when a mechanical switch is
  opened or closed

Figure 11-35
Figure 11-34
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Switch Debouncing

• Schmitt method of debouncing a SPST switch

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Switch Debouncing

• Cross-NAND method of debouncing a SPDT switch

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Switch Debouncing

• D flip-flop method of debouncing a SPDT switch

Figure 11-38
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Sizing Pull-Up Resistors

• Used to prevent floating
• Avoid high power consumption
• 10 k? resistor works fine
• Pull-down resistor
• To keep terminal LOW
• 100 ? resistor works fine

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Practical Input and Output Considerations

• 5 V Power Supply
• 78XX series integrated circuit voltage regulators
• ac-to-dc rectifier circuit
• Ripple-free output

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Practical Input and Output Considerations

• 60 Hz clock

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Practical Input and Output Considerations

• Driving Light-Emitting Diodes
• Provides current path (sink) when output is LOW
• 330 ? resistor limits current

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Practical Input and Output Considerations

• Phototransistor Input to a Latching Alarm

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Practical Input and Output Considerations

• Using an Optocoupler for Level Shifting

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Practical Input and Output Considerations

• Event counting with an optical interrupter switch

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Figures 11-46 and 11-47
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Practical Input and Output Considerations

• Power MOSFET to Drive a Relay

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Practical Input and Output Considerations

• Level Detecting with an LM339 Analog Comparator

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Practical Input and Output Considerations

• Using a Hall-effect switch as a digital input

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Summary

• Unpredictable results on IC logic can occur if
  strict timing requirements are not met.
• A setup time is required to ensure that the input
  data to a logic circuit is present some defined
  time prior to the active clock edge.
• A hold time is required to ensure that the input
  data to a logic circuit is held for some definite
  time after the active clock edge.

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Summary

• The propagation delay is the length of time it
  takes for the output of a logic circuit to
  respond to an input stimulus.
• Delay gates are available to purposely introduce
  time delays when required.
• The charging voltage on a capacitor in a series
  RC circuit can be used to create a short delay
  for a power-up reset.

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Summary

• The two key features of Schmitt trigger ICs are
  that they output extremely sharp edges and they
  have two distinct input threshold voltages. The
  difference between the threshold voltages is
  called the hysteresis voltage.
• Mechanical switches exhibit a phenomenon called
  switch bounce, which can cause problems in most
  kinds of logic circuits.

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Summary

• Pull-up resistors are required to make a normally
  floating input act like a HIGH. Pull-down
  resistors are required to make a normally
  floating input act like a LOW.
• A practical, inexpensive 5 V power supply can be
  made with just a transformer, four diodes, some
  capacitors, and a voltage regulator.

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Summary

• A 60-pulse-per-second clock oscillator can be
  made using the power supplys transformer and a
  few additional components.
• The resistance from collector to emitter of a
  phototransistor changes from about 10 M? down to
  about 1 k? when light shines on its base region.

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Summary

• An optocoupler provides electrical isolation from
  one part of a circuit to another.
• Power MOSFETs are commonly used to increase the
  output drive capability of IC logic from less
  than 100 mA to more than 1A.

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