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)