Practical Considerations for Digital Design Chapter 11 Practical Considerations for Digital Design 1

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

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

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

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

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

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

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

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

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

Automatic Reset RC circuit to reset at power-up 11

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

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

Schmitt Trigger ICs Transfer Function 14

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

Switch Debouncing Schmitt method of debouncing a SPST switch 16

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

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

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

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

Practical Input and Output Considerations 60 Hz clock 21

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

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

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

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

Figures 11-46 and 11-47 26

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

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

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

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

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

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

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

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

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