Computers as Components, 2nd Edition

4.8 Design Example: Alarm Clock 203

Time

Button

event

Button

input

FIGURE 4.40

Preprocessing button inputs.

button values so that the user interface code will respond properly. The buttons

will remain depressed for many sample periods since the sample rate is much faster

than any person can push and release buttons. We want to make sure that the clock

responds to this as a single depression of the button,not one depression per sample

interval. As shown in Figure 4.40, this can be done by performing a simple edge

detection on the button input—the button event value is 1 for one sample period

when the button is depressed and then goes back to 0 and does not ...