import time
from machine import Pin, PWM

led = PWM(Pin(0))
led.freq(1000) #1000 cycles per second

x = 0 #duty cycle

#duty cycle has to be between 0 to 65535 that is 16 bit unsigned integer

while True:
    led.duty_u16(x)
    time.sleep(0.6)
    x += 20000

    if x > 65535:
        x = 0