EBC Exercise 13 Pulse Width Modulation

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In a previous exercise (EBC Exercise 03 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 we will learn how to use the Beagle's pulse width modulation (pwm) hardware by writing directly to the registers that control it and also learn about pin multiplexing (pin mux) on the way.

PWM and pin MUXing

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.

Assignment

Updated pwd files are here.

  1. Copy the dm3730-demo directory to your Beagle.
  2. Look at the file to see what they are doing.
  3. Run make, then pwd-demo.
  4. Hook up a oscilloscope. (See table 20 of the Beagle System Reference manual to see where to probe.) Are the pwd outputs doing what you expected?
  5. What's the highest frequency you can generate? What's the lowest?
  6. Create a new C program, based on pwm-demo, that take 3 parameters, the <pwm to use>, <frequency> and <duty cycle>.
  7. 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.

Resources

  1. BeagleBoardPWM from ECE597
  2. BeagleBoard/GSoC/2010_Projects/Pulse_Width_Modulation Google SoC project
  3. BeagleBoardPinMux, how to set the pin mux.