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Chapter 11 Practical Considerations for Digital Design 1.

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Presentation on theme: "Chapter 11 Practical Considerations for Digital Design 1."— Presentation transcript:

1 Chapter 11 Practical Considerations for Digital Design 1

2 Objectives You should be able to: Describe the causes and effects of a race condition on synchronous flip-flop operation. 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. Use manufacturers’ data sheets to determine IC operating specifications such as setup time, hold time, propagation delay, and input/output voltage and current specifications. 2

3 Objectives (Continued) Perform worst-case analysis on the time-dependent operations of flip-flops and sequential circuitry. 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 Design a series RC circuit to provide an automatic power-up reset function 3

4 Objectives (Continued) Describe the wave-shaping capability and operating characteristics of Schmitt trigger ICs. 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. Describe the problems caused by switch bounce and how to eliminate its effects. Calculate the optimum size for a pull-up resistor. Calculate the optimum size for a pull-up resistor. 4

5 Flip-Flop Time Parameters Race condition – inputs are changing at the same time as the active clock edge Race condition – inputs are changing at the same time as the active clock edge 5

6 Flip-Flop Time Parameters Data Manual provides ac waveforms to illustrate measuring points Data Manual provides ac waveforms to illustrate measuring points Setup time – how far back the FF looks to determine input levels Setup time – how far back the FF looks to determine input levels 6

7 Flip-Flop Time Parameters Hold time – how long the input level must be held beyond the active clock edge Hold time – how long the input level must be held beyond the active clock edge 7

8 Flip-Flop Time Parameters Propagation delay - delay from input to output Propagation delay - delay from input to output 8

9 Flip-Flop Time Parameters Other information in Manufacturer’s Data Manual: Other information in Manufacturer’s Data Manual: Maximum frequency (f max ) Maximum frequency (f max ) Clock pulse width [t w (L)] and [t w (H)] Clock pulse width [t w (L)] and [t w (H)] Set or reset pulse width [t w (L)] Set or reset pulse width [t w (L)] Metastable state Metastable state Output voltage at an invalid level Output voltage at an invalid level Delay gate ICs provide exact delays Delay gate ICs provide exact delays 9

10 Discussion Point Locate the following items on the sample data sheet shown in Figure 11-11: Locate the following items on the sample data sheet shown in Figure 11-11: Pin configuration Pin configuration Function table Function table Propagation delay Propagation delay Setup and hold times Setup and hold times 10

11 Automatic Reset RC circuit to reset at power-up RC circuit to reset at power-up 11

12 Schmitt Trigger ICs Transforms slowly changing waveforms to sharply defined output signals Transforms slowly changing waveforms to sharply defined output signals 12

13 Schmitt Trigger ICs Positive feedback is used to speed up level transitions Positive feedback is used to speed up level transitions Hysteresis Hysteresis Switching thresholds of positive and negative-going signals are different Switching thresholds of positive and negative-going signals are different Useful to ignore small amounts of jitter Useful to ignore small amounts of jitter 13

14 Schmitt Trigger ICs Transfer Function Transfer Function 14

15 Switch Debouncing Switch bounce occurs when a mechanical switch is opened or closed Switch bounce occurs when a mechanical switch is opened or closed Figure Figure

16 Switch Debouncing Schmitt method of debouncing a SPST switch Schmitt method of debouncing a SPST switch 16

17 Switch Debouncing Cross-NAND method of debouncing a SPDT switch Cross-NAND method of debouncing a SPDT switch 17

18 Switch Debouncing D flip-flop method of debouncing a SPDT switch D flip-flop method of debouncing a SPDT switch Figure

19 Sizing Pull-Up Resistors Used to prevent floating Used to prevent floating Avoid high power consumption Avoid high power consumption 10 k  resistor works fine 10 k  resistor works fine Pull-down resistor Pull-down resistor To keep terminal LOW To keep terminal LOW 100  resistor works fine 100  resistor works fine 19

20 Practical Input and Output Considerations 5 V Power Supply 5 V Power Supply 78XX series integrated circuit voltage regulators 78XX series integrated circuit voltage regulators ac-to-dc rectifier circuit ac-to-dc rectifier circuit Ripple-free output Ripple-free output 20

21 Practical Input and Output Considerations 60 Hz clock 60 Hz clock 21

22 Practical Input and Output Considerations Driving Light-Emitting Diodes Driving Light-Emitting Diodes Provides current path (sink) when output is LOW Provides current path (sink) when output is LOW 330  resistor limits current 330  resistor limits current 22

23 Practical Input and Output Considerations Phototransistor Input to a Latching Alarm Phototransistor Input to a Latching Alarm 23

24 Practical Input and Output Considerations Using an Optocoupler for Level Shifting Using an Optocoupler for Level Shifting 24

25 Practical Input and Output Considerations Event counting with an optical interrupter switch Event counting with an optical interrupter switch 25

26 Figures and

27 Practical Input and Output Considerations Power MOSFET to Drive a Relay Power MOSFET to Drive a Relay 27

28 Practical Input and Output Considerations Level Detecting with an LM339 Analog Comparator Level Detecting with an LM339 Analog Comparator 28

29 Practical Input and Output Considerations Using a Hall-effect switch as a digital input Using a Hall-effect switch as a digital input 29

30 Summary Unpredictable results on IC logic can occur if strict timing requirements are not met. 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 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. 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. 30

31 Summary The propagation delay is the length of time it takes for the output of a logic circuit to respond to an input stimulus. 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. 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. The charging voltage on a capacitor in a series RC circuit can be used to create a short delay for a power-up reset. 31

32 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. 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. Mechanical switches exhibit a phenomenon called switch bounce, which can cause problems in most kinds of logic circuits. 32

33 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. 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. A practical, inexpensive 5 V power supply can be made with just a transformer, four diodes, some capacitors, and a voltage regulator. 33

34 Summary A 60-pulse-per-second clock oscillator can be made using the power supply’s transformer and a few additional components. A 60-pulse-per-second clock oscillator can be made using the power supply’s 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. 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. 34

35 Summary An optocoupler provides electrical isolation from one part of a circuit to another. 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. Power MOSFETs are commonly used to increase the output drive capability of IC logic from less than 100 mA to more than 1A. 35


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