from machine import Pin,PWM
from time import sleep

pwm=PWM (Pin(9))
pwm.freq(1000)

while True:
     for duty in range(65535):
        pwm.duty_u16(duty)
        sleep(0.0001)
     sleep(0.5)
     for duty in range(65535, 0, -1):
        pwm.duty_u16(duty)
        sleep(0.0001)
    sleep(0.5)