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EBC Exercise 13 Pulse Width Modulation

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thumb‎ Embedded Linux Class by Mark A. Yoder

3.8 Kernel

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.

PWM on the Bone

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

The Bone has a PWM interface at /sys/class/pwm/. You can see what's there by:

beagle$ cd /sys/class/pwm
beagle$ ls -F
export  unexport

Hmmm, there isn't much there. We have to run a command to make something appear. Try

beagle$ SLOTS=/sys/devices/bone_capemgr.*/slots
beagle$ PINS=/sys/kernel/debug/pinctrl/44e10800.pinmux/pins
beagle$ echo am33xx_pwm > $SLOTS
beagle$ ls -F
export  pwmchip0@  pwmchip2@  pwmchip3@  pwmchip5@  pwmchip7@  unexport

Now we need to run another command to say which pwm pin we want to use. I'm using P9_21.

beagle$ echo bone_pwm_P9_21 > $SLOTS

Now you can export a pwm much list you export a gpio port

beagle$ echo 1 > export
beagle$ cd pwm1
beagle$ ls -F
device@  duty_ns  period_ns  polarity  power/  run  subsystem@  uevent

Try a 1kHz frequency with a 25% duty cycle

beagle$ echo 1000000 > period_ns
beagle$ echo  250000 > duty_ns
beagle$ echo 1 > run

If you have an oscilloscope try probing pin P9_21. I'm getting a nice clean 1kHz 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.

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?

Stick a scope on the pin and see if the frequency and duty cycle are right. What's the highest frequency you can get? What's the lowest?

thumb‎ Embedded Linux Class by Mark A. Yoder