Pin 9 generates a PWM signal using a time-honored technique. Suppose you want to generate 20/255 (about 8% duty cycle). You take an 8-bit accumulator and add 20 to it repeatedly.

Pulse-width modulation (PWM) Pulse width modulation (PWM) is a powerful technique for controlling analog circuits with a microprocessor’s digital outputs. Pulse-width modulation,as it applies to LED ...