BeagleBoard/GSoC/2010 Projects/Pulse Width Modulation

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Project: Pulse Width Modulation

Student: Varun Jewalikar

Mentors: Søren Steen Christensen, Cristina Murillo




The main aim of this project will be to provide a high level interface for the Pulse Width Modulation output pins of the OMAP3 found on the BeagleBoard. Another aim will be to push this driver upstream(the Linux-OMAP tree). Two applications will be developed to demonstrate the use of this driver:

  • Motor control using PWM signals generated by the BeagleBoard using the PWM driver.
  • Writing a glue layer for ALSA to communicate with the PWM driver and output sound from the PWM output pins.

Build and Run instructions

  • Clone my git tree for the driver.This can be done using the following commands:
git clone git://
cd omap3-pwm/
  • For newer kernel versions, you can try the fork of the original project (should work with the BeagleBone).
git clone git://
cd omap3-pwm
  • A cross compiling environment will be needed for compiling the driver. Instructions below work with OpenEmbedded but can adapted for codesourcery as well.
$ vim beagle-source-me.txt

Edit the OETREE path. OETREE should point to the OE folder. For me OETREE= ~/OE .

$ source beagle-source-me.txt

This will set the environment variables for cross compilation.

$ make 

This will generate the requisite pwm.ko kernel module file.

  • Copy the pwm.ko to your target board.
  • Once on the system, use insmod to load using the optional frequency parameter. The default frequency is 1024 Hz. Use multiples of two with a max of 16384. pwm9_enable,pwm10_enable and pwm11_enable and frequency are load time parameters.
root@beagleboard# ls
root@beagleboard# insmod pwm.ko pwm9_enable=1

The driver implements a character device interface. When it loads, it will create a /dev/pwm9 entry.

  • To setup multiple pwm signals on the GPT9/10/11 you should use
root@beagleboard# insmod pwm.ko pwm9_enable=1 pwm10_enable=1

This will work similarly for any other (pwm_9/10/11) combinations as well.

  • There are two ways of modifying the parameters from userland.

Using the sysfs interface (Not advised and fully supported) : Then to issue commands you can use any program that can do file I/O cat and echo will work.

root@beagleboard# cat /dev/pwm9
PWM9 Frequency 1024 Hz Stopped
root@beagleboard# echo 50 > /dev/pwm9
root@beagleboard:~# cat /dev/pwm9
PWM9 Frequency 1024 Hz Duty Cycle 50%
root@beagleboard:~# echo 80 > /dev/pwm9
root@beagleboard:~# cat /dev/pwm9
PWM9 Frequency 1024 Hz Duty Cycle 80%

You can put an oscope on pin 28 of the expansion board to see the signal. Use pin 15 for ground. Or you can measure the voltage on pin 28 and you'll see the duty cycle percentage of 1.8v.

  • Using the ioctl() calls :

A fully supported ioctl() interface is available and it is advised to prefer this over the above sysfs interface. Sample code to use the ioctl interface:

#include <termios.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <linux/ioctl.h>
//#define PWM_IOC_MAGIC  0x00 
#define PWM_SET_DUTYCYCLE 1074003969
#define PWM_GET_DUTYCYCLE 1074003970
#define PWM_SET_FREQUENCY 1074003971
#define PWM_GET_FREQUENCY 1074003972
#define PWM_ON 1074003973
#define PWM_OFF 1074003974
#define PWM_SET_POLARITY 1074003975
	int fd, status, k, f, sc;

	fd = open("/dev/pwm9", O_RDWR);
	scanf("%d", &k);
	scanf("%d", &f);
	scanf("%d", &sc);

	if (ioctl(fd, PWM_SET_FREQUENCY, f) == -1)
		printf("TIOCMGET failed: %s\n", strerror(errno));

	if (ioctl(fd, PWM_SET_DUTYCYCLE, k) == -1)
		printf("TIOCMGET failed: %s\n", strerror(errno));

	if (ioctl(fd, PWM_SET_POLARITY, sc) == -1)
		printf("TIOCMGET failed: %s\n", strerror(errno));
	printf("%d",ioctl(fd, PWM_GET_FREQUENCY, sc));
	printf("\n%d",ioctl(fd, PWM_GET_DUTYCYCLE, sc));

  • For more information refer to the README file. [1]

Kernel Patch

The kernel config option CONFIG_OMAP_RESET_CLOCKS is enabled in the default beagleboard defconfigs. You'll get an oops using pwm.ko with this enabled. This is a kernel power saving feature. You'll need to disable this config option to use this driver. Below is a sample patch for linux-omap-2.6.32's defconfig. Adjust for the kernel you are using. Gumstix users already have this turned off in default kernels.

diff --git a/recipes/linux/linux-omap-2.6.32/beagleboard/defconfig b/recipes/linux/linux-omap-2.6.32/beagleboard/defconfig
index cebe1f5..2dad30c 100644
--- a/recipes/linux/linux-omap-2.6.32/beagleboard/defconfig
+++ b/recipes/linux/linux-omap-2.6.32/beagleboard/defconfig
@@ -241,7 +241,7 @@ CONFIG_ARCH_OMAP3=y
 # CONFIG_OMAP_MUX is not set