1. Practical Considerations for Digital Design
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.
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.
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 problems caused by switch bounce and how to eliminate its effects.
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 5

6. 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 6

7. Flip-Flop Time Parameters
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 8

9. Flip-Flop Time Parameters
Other information in Manufacturer’s 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 9

10. 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 10

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

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

13. 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 13

14. Schmitt Trigger ICs
Transfer Function 14

15. Switch Debouncing
Switch bounce occurs when a mechanical switch is opened or closed
Figure 11-35
Figure 11-34 15

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

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

18. Switch Debouncing D flip-flop method of debouncing a SPDT switch
Figure 11-38 18

19. 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 19

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

21. Practical Input and Output Considerations
60 Hz clock 21

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

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

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

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

26. Figures and 11-47 26

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

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

29. Practical Input and Output Considerations
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.
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. 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.
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. 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.
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.
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.
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.
Power MOSFETs are commonly used to increase the output drive capability of IC logic from less than 100 mA to more than 1A. 35