Chapter 11 Practical Considerations for Digital Design 1.

Chapter 11Chapter 11

Practical Considerations for Practical Considerations for Digital DesignDigital Design

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ObjectivesObjectives

You should be able to:You should be able to: Describe the causes and effects of a Describe the causes and effects of a

race condition on synchronous flip-flop race condition on synchronous flip-flop operation.operation.

Use manufacturers’ data sheets to Use manufacturers’ data sheets to determine IC operating specifications determine IC operating specifications such as setup time, hold time, such as setup time, hold time, propagation delay, and input/output propagation delay, and input/output voltage and current specifications.voltage and current specifications.

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(Continued)(Continued) Perform worst-case analysis on the Perform worst-case analysis on the

time-dependent operations of flip-flops time-dependent operations of flip-flops and sequential circuitry.and sequential circuitry.

Design a series RC circuit to provide an Design a series RC circuit to provide an automatic power-up reset functionautomatic power-up reset function

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(Continued)(Continued) Describe the wave-shaping capability Describe the wave-shaping capability

and operating characteristics of Schmitt and operating characteristics of Schmitt trigger ICs.trigger ICs.

Describe the problems caused by switch Describe the problems caused by switch bounce and how to eliminate its effects.bounce and how to eliminate its effects.

Calculate the optimum size for a pull-up Calculate the optimum size for a pull-up resistor.resistor.

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Flip-Flop Time Flip-Flop Time ParametersParameters

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

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Data Manual provides ac Data Manual provides ac waveforms to illustrate measuring waveforms to illustrate measuring pointspoints

Setup time – how far back the FF Setup time – how far back the FF looks to determine input levelslooks to determine input levels

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Hold time – how long the input level Hold time – how long the input level must be held beyond the active clock must be held beyond the active clock edgeedge

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Propagation delay - delay from input to Propagation delay - delay from input to outputoutput

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Other information in Manufacturer’s Other information in Manufacturer’s Data Manual:Data Manual: Maximum frequency (fMaximum frequency (fmaxmax)) Clock pulse width [tClock pulse width [tw w (L)] and [t(L)] and [tw w (H)] (H)] Set or reset pulse width [tSet or reset pulse width [tw w (L)] (L)]

Metastable stateMetastable state Output voltage at an invalid levelOutput voltage at an invalid level

Delay gate ICs provide exact delaysDelay gate ICs provide exact delays

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Discussion PointDiscussion Point

Locate the following items on the Locate the following items on the sample data sheet shown in Figure sample data sheet shown in Figure 11-11:11-11: Pin configurationPin configuration Function tableFunction table Propagation delayPropagation delay Setup and hold timesSetup and hold times

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Automatic ResetAutomatic Reset RC circuit to reset at power-upRC circuit to reset at power-up

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Schmitt Trigger ICsSchmitt Trigger ICs Transforms slowly changing waveforms to Transforms slowly changing waveforms to

sharply defined output signalssharply defined output signals

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Schmitt Trigger ICsSchmitt Trigger ICs

Positive feedback is used to speed up Positive feedback is used to speed up level transitionslevel transitions

HysteresisHysteresis Switching thresholds of positive and Switching thresholds of positive and

negative-going signals are differentnegative-going signals are different Useful to ignore small amounts of jitterUseful to ignore small amounts of jitter

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Schmitt Trigger ICsSchmitt Trigger ICs Transfer FunctionTransfer Function

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Switch DebouncingSwitch Debouncing Switch bounce occurs when a Switch bounce occurs when a

mechanical switch is opened or mechanical switch is opened or closedclosed

Figure 11-34

Figure 11-35

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Switch DebouncingSwitch Debouncing Schmitt method of debouncing a Schmitt method of debouncing a

SPST switchSPST switch

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Switch DebouncingSwitch Debouncing Cross-NAND method of debouncing a Cross-NAND method of debouncing a

SPDT switchSPDT switch

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Switch DebouncingSwitch Debouncing D flip-flop method of debouncing a D flip-flop method of debouncing a

SPDT switchSPDT switch

Figure 11-38

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Sizing Pull-Up ResistorsSizing Pull-Up Resistors

Used to prevent floatingUsed to prevent floating Avoid high power consumptionAvoid high power consumption 10 k10 k resistor works fine resistor works fine

Pull-down resistorPull-down resistor To keep terminal LOWTo keep terminal LOW 100 100 resistor works fine resistor works fine

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Practical Input and Output Practical Input and Output ConsiderationsConsiderations

5 V Power Supply5 V Power Supply 78XX series integrated circuit voltage 78XX series integrated circuit voltage

regulatorsregulators ac-to-dc rectifier circuitac-to-dc rectifier circuit Ripple-free outputRipple-free output

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Practical Input and Practical Input and Output ConsiderationsOutput Considerations

60 Hz clock60 Hz clock

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Driving Light-Emitting DiodesDriving Light-Emitting Diodes Provides current path (sink) when output is Provides current path (sink) when output is

LOWLOW 330 330 resistor limits current resistor limits current

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Phototransistor Input to a Phototransistor Input to a Latching AlarmLatching Alarm

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Using an Optocoupler for Level Using an Optocoupler for Level ShiftingShifting

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Event counting with an optical Event counting with an optical interrupter switchinterrupter switch

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Figures 11-46 Figures 11-46 and 11-47and 11-47

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Power MOSFET to Drive a RelayPower MOSFET to Drive a Relay

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Level Detecting with an LM339 Level Detecting with an LM339 Analog ComparatorAnalog Comparator

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Using a Hall-effect switch as a Using a Hall-effect switch as a digital inputdigital input

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SummarySummary

Unpredictable results on IC logic can occur Unpredictable results on IC logic can occur if strict timing requirements are not met.if strict timing requirements are not met.

A setup time is required to ensure that the A setup time is required to ensure that the input data to a logic circuit is present some input data to a logic circuit is present some defined time prior to the active clock edge.defined time prior to the active clock edge.

A hold time is required to ensure that the A hold time is required to ensure that the input data to a logic circuit is held for some input data to a logic circuit is held for some definite time after the active clock edge.definite time after the active clock edge.

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SummarySummary

The propagation delay is the length of The propagation delay is the length of time it takes for the output of a logic time it takes for the output of a logic circuit to respond to an input stimulus.circuit to respond to an input stimulus.

Delay gates are available to purposely Delay gates are available to purposely introduce time delays when required.introduce time delays when required.

The charging voltage on a capacitor in a The charging voltage on a capacitor in a series RC circuit can be used to create a series RC circuit can be used to create a short delay for a power-up reset.short delay for a power-up reset.

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SummarySummary

The two key features of Schmitt The two key features of Schmitt trigger ICs are that they output trigger ICs are that they output extremely sharp edges and they have extremely sharp edges and they have two distinct input threshold voltages. two distinct input threshold voltages. The difference between the threshold The difference between the threshold voltages is called the hysteresis voltages is called the hysteresis voltage.voltage.

Mechanical switches exhibit a Mechanical switches exhibit a phenomenon called switch bounce, phenomenon called switch bounce, which can cause problems in most which can cause problems in most kinds of logic circuits.kinds of logic circuits.

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SummarySummary

Pull-up resistors are required to Pull-up resistors are required to make a normally floating input act make a normally floating input act like a HIGH. Pull-down resistors are like a HIGH. Pull-down resistors are required to make a normally floating required to make a normally floating input act like a LOW.input act like a LOW.

A practical, inexpensive 5 V power A practical, inexpensive 5 V power supply can be made with just a supply can be made with just a transformer, four diodes, some transformer, four diodes, some capacitors, and a voltage regulator.capacitors, and a voltage regulator.

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SummarySummary

A 60-pulse-per-second clock oscillator A 60-pulse-per-second clock oscillator can be made using the power supply’s can be made using the power supply’s transformer and a few additional transformer and a few additional components.components.

The resistance from collector to The resistance from collector to emitter of a phototransistor changes emitter of a phototransistor changes from about 10 Mfrom about 10 M down to about 1 k down to about 1 k when light shines on its base region.when light shines on its base region.

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SummarySummary

An optocoupler provides electrical An optocoupler provides electrical isolation from one part of a circuit to isolation from one part of a circuit to another.another.

Power MOSFETs are commonly used Power MOSFETs are commonly used to increase the output drive to increase the output drive capability of IC logic from less than capability of IC logic from less than 100 mA to more than 1A.100 mA to more than 1A.

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Chapter 11 Practical Considerations for Digital Design 1.

Documents

switch debouncing d

mechanical switch

time parameters data

input levels setup time

spdt switch figure

active clock edge hold

spdt switch d

closed switch bounce