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This sample program uses the PWM device to control the output level of GPIO 18
and 19 (GPIO 19 not available on Raspberry Pi 1 Model A and B). If there is a
LED connected (via a resistor) to one of these pins its brightness changes
continuously. The state of GPIO 19 is opposed to that of GPIO 18. When the LED
on GPIO 18 is bright, the LED on GPIO 19 is dark.
