EBC Exercise 13 Pulse Width Modulation

This is for the 3.8 kernel. EBC Exercise 13 Pulse Width Modulation 3.2 is for the 3.2 kernel.

In a previous exercise (EBC Exercise 11 gpio Polling and Interrupts) you saw how to use the gpio to produce a square wave out using a C program and sysfs. I was able to get a 1.5kHz square wave out; however we can do much better using some built in hardware on the Beagle.

In this exercise you will learn how to use the Beagle's pulse width modulation (pwm) hardware using the sysfs interface.

Pulse Width Modulation
(Note: The pwm interface seems to changing. Some of this may not apply in the future.)

We have to run a couple of commands to make the PWM interface appear. Try bone$ SLOTS=/sys/devices/bone_capemgr.*/slots bone$ echo am33xx_pwm    > $SLOTS bone$ echo bone_pwm_P9_21 > $SLOTS bone$ cd /sys/devices/ocp.2/pwm_test_P9_21.14 bone$ ls driver duty  modalias  period  polarity  power  run  subsystem  uevent

The units are in ns. Try a 1Hz frequency with a 25% duty cycle bone$ echo 1000000000 > period bone$ echo 250000000 > duty bone$ echo 1 > run

Connect the LED from and watch it flash. Try changing the frequency and duty cycle. You may have to set the duty cycle to 0 to change the frequency. Can you guess why?

Challenge
Combine the analog in and the PWM by having the pot control the frequency or the duty cycle of the LED.

If you have an oscilloscope try probing pin P9_21. I'm getting a nice clean 1Hz signal, with no variation. Let's try a higher frequency, like 10 MHz. beagle$ echo 50 > duty_ns beagle$ echo 100 > period_ns I'm getting a 9 MHz signal that has lots of ringing. The timer in the bone must be off a bit.

The AM335x PWM Driver's Guide details what eCAP and eHRPWM are and gives some examples.