May 2005
Beginner to intermediate
398 pages
12h 12m
English
Content preview from Designing Embedded Hardware, 2nd Edition
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PWM
Using a DAC may seem the obvious way to generate an analog output voltage, but there is another way that uses nothing more than a digital I/O line configured as an output. This technique is known as Pulse Width Modulation (PWM).
Consider the average, garden-variety, square wave shown in Figure 13-26.
Figure 13-26. A ubiquitous square wave
The width of the high is equal to the width of the low, so this wave is said to have a 50% duty cycle. In other words, it is high for exactly half the cycle. Now, if the amplitude of this square wave is 5 V, for example, the average voltage over the cycle is 2.5 V. It is as though we had a constant voltage of 2.5 V.
Now consider the square wave in Figure 13-27.
Figure 13-27. 10% duty cycle
This wave has a 10% duty cycle, which means that the average voltage over the cycle is 0.5 V.
A low-pass (averaging) filter on the PWM output will convert the pulses to an analog voltage, proportional to the duty cycle of the PWM signal. By varying the duty cycle, we can vary the analog voltage. Hey, presto! We have digital-to-analog conversion without a DAC. That's the basic idea behind PWM.
Tip
PWM can also be used to drive a LED, and thereby get varying light intensities from a signal that is essentially either on or off. PWM can also be used to generate audio. Early ...
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