from machine import Pin, PWM
from time import sleep

led = Pin(0, Pin.OUT)

pwm = PWM(led)
pwm.freq(1000)

while True:
    for duty in range(0,65536,1000):
        pwm.duty_u16(duty)
        sleep(0.05)
    
    for duty in range(65535,0,-1000):
        pwm.duty_u16(duty)
        sleep(0.05)