from machine import Pin, PWM
from time import sleep   

# wait for USB to become ready
sleep(0.1)

# setup PWM on GP15
led = PWM(Pin(15))
led.freq(1000)
#------------------main----------------
while True:
    # Fade IN (0 ->100%)
    for duty in range(0, 65536, 1000)
    led.duty_u16(duty)
    sleep(0.02)

    # Fade OUT(100% ->0)
    for duty in range (65535, -1,-1000)
    led.duty_u16(duty)
    sleep(0.02)