from machine import Pin, PWM
import time 

led = PWM(Pin(15)) 
led.freq(1000)

while True:
    for d in range(0, 65536, 1024):
        led.duty_u16(d)
        time.sleep_ms(10)
    for d in range(65535, -1, -1024):
        led.duty_u16(d)
        time.sleep_ms(10)