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Some computer controlled devices requires a pulse width modulated signal. In the digital world, an output is either low or high. If the output device has only one pin, pulse width modulation is a convenient way to represent a large range of output values. A pulse width modulated signal is a sequence of pulses characterized by frequency and duty cycle. Frequency describes how often the signal repeats. Duty cycle describes the shape of the signal within a single period. Referring to the slide, the duty cycle is defined as (w/u)x100%, so a 30% duty cycle means within one period of length u, the signal is high 30 percent of the time and low the remaining 70 percent of the time.
Microprocessors can easily generate PWM signals with any desired duty cycle using either hardware or software. Note that when the duty cycle of a signal changes, the average voltage of the signal changes. Operation of some external devices depends on the average input voltage; e.g. the speed of a DC motor is proportional to the average input voltage.