I'm running the latest 5.10.21-sunxi kernel. I figured out how to use the PWM pin with the help of this post.

 

I have an LED connected to the PWM pin and when I run for example:

sudo sh -c "echo 250 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle"

the LED updates instantly. But when I try to fade the LED with a script, the LED always takes 6 seconds to update.

 

This is the script:

#!/bin/bash

#Export PWM channel for user control.
sh -c "echo 0 > /sys/class/pwm/pwmchip0/export"

#Select the period of PWM signal. Value is in nanoseconds.
sh -c "echo 500 > /sys/class/pwm/pwmchip0/pwm0/period"

#Change the polarity of the PWM signal.
#The polarity can only be changed if the PWM is not enabled.
sh -c "echo "normal" > /sys/class/pwm/pwmchip0/pwm0/polarity"

#Enable the PWM signal.
sh -c "echo 1 > /sys/class/pwm/pwmchip0/pwm0/enable"

for i in {0..500}
do
        sh -c "echo $i > /sys/class/pwm/pwmchip0/pwm0/duty_cycle"
        cat /sys/class/pwm/pwmchip0/pwm0/duty_cycle #Just for debugging
        sleep 0.1
done

#Disable the PWM signal.
sh -c "echo 0 > /sys/class/pwm/pwmchip0/pwm0/enable"

#UnExport PWM channel for user control.
sh -c "echo 0 > /sys/class/pwm/pwmchip0/unexport"

 

I hope someone can help me. It's so hard to find any resources for this sort of thing.

Ok, this was a pretty dumb post. This example that I found seems to be incorrect. The value for the period isn't in nanoseconds, but in picoseconds, so the correct value would actually be 500000. 500 picosecond period would mean a frequency of 2 MHz, which is way too high. The problem wasn't that the LED took too long to update, but that the resolution at this frequency was way too low.