EBC Exercise 13 Pulse Width Modulation
Embedded Linux Class by Mark A. Yoder
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 and also learn about pin multiplexing (pin mux) on the way.
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?
PWM on the xM
This section needs updating to use /sys/kernel/debug/omap_mux to get the pin MUXes.
The DM3730 has 11 general purpose timers, 4 of which (gpt8-gpt11) can be brought out of the chip and used for pulse width modulation (DM3730 TRM page 2689). The problem is the DM3730 has more internal lines than hardware I/O pins. The solution is that I/O pins run though a MUX that selects which internal lines appear on I/O pins. A given pin can have one from as many as eight lines assigned to it.
These MUXes are set at boot time, and must be set when the kernel boots, or in u-boot. I couldn't set them during kernel boot with the 2.6.32 kernel, so I used u-boot. BeagleBoardPinMux is a good place to learn about the pin MUXing. The u-boot details are here.
BeagleBoardPWM is a nice overview of how to do PWM on the Beagle. The version of the kernel and u-boot that I've given you should already be configured to access the PWM pins. If it isn't you'll have to recompile the Kernel and u-boot.
The standard way to interface with the outside world in Linux is through Kernel Drivers. Currently there are no standard PWM driver for the Beagle, though a couple have been proposed (,  and ). BeagleBoardPWM takes a more traditional MCU approach by accessing the memory mapped PWD registers directly using mmap in a C program. Although this approach works, it is really transitional until a standard can be established.
You could even do PWM from a shell command by using devmem2 to write to the memory mapped registers from a command line.
Here's another PWM lead.
If your git repository is set up just:
beagle$ cd exercises beagle$ git pull beagle$ cd pwm
(Follow the instructions here if you aren't set up for git.)
- Look at the files to see what they are doing.
- Run make, then pwm-demo.
- Hook up a oscilloscope. (See Table 22 of the Beagle System Reference manual to see where to probe.) Are the pwm outputs doing what you expected?
- What's the highest frequency you can generate? What's the lowest?
- Create a new C program, based on pwm-demo, that takes 3 parameters, the <pwm to use>, <frequency> and <duty cycle>.
- Create a shell file that will call your new program and set up the three pwm's that appear on the expansion header and program them to do something interesting.
- Write a shell file that will do the pin MUXing using devmem2.
- Rewrite pwm-demo as a shell file that uses devmem2.
- BeagleBoardPWM from ECE597
- BeagleBoard/GSoC/2010_Projects/Pulse_Width_Modulation Google SoC project
- BeagleBoardPinMux, how to set the pin mux.
- Buttons and PWM
- Shaky PWMs
- PWM on the bone
Embedded Linux Class by Mark A. Yoder