A driver for the PWM timers of an OMAP3 based Linux system exposing a simple char dev interface to userland.
Jack Elston and Curtis Olson are coathors of this code. They are responsible for the [four-channel] branch of this project.
The TI TRM is the primary reference for the code.
I put up some of my notes for the OMAP3 PWM timers here:
http://www.jumpnowtek.com/index.php?option=com_content&view=article&id=56&Itemid=63
And Curtis Olson has a more general PWM article here:
http://gallinazo.flightgear.org/technology/gumstix-overo-rc-servos-and-pwm-signal-generation/
The code should work with any OMAP3 board, but I only tested with Gumstix Overo and Beagleboard.
The [master] branch of this project only implements one PWM channel. The default is PWM10.
The [four-channel] branch of the project implements all 4 PWM channels each with their own char dev node.
The [explicit-clk-enable] branch is for 2.6.39 kernels and later. Previous kernels had the PWM source clocks enabled by default if CONFIG_OMAP_RESET_CLOCKS was not enabled. No longer, so the driver code now does it explicitly.
There is a ${MACHINE}-source-me.txt file that will set up your environment for the cross-compilation. It assumes you are using an OE environment and it tries to be generic enough for both userland and kernel/module stuff.
You should modify or create a similar script for pointing to the build system you are using.
If you modified your OE temp directory, then also update the OETMP variable in the appropriate ${MACHINE}-source-me.txt. I kind of tested overo and beagleboard, but I don't normally use the defaults.
Follow these steps to build. Using an overo for the example.
$ git clone git://github.com/scottellis/omap3-pwm.git
$ cd omap3-pwm
If you want to build the [four-channel] branch use git to check it out now.
$ git checkout -b four-channel origin/four-channel
Then
$ <edit> overo-source-me.txt
$ source overo-source-me.txt
$ make
Next copy the pwm.ko file to your board.
These final instructions apply to the [master] branch version of the driver.
The [four-channel] instruction are coming soon.
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.
root@overo# ls
pwm.ko
root@overo# insmod pwm.ko
The driver implements a character device interface. When it loads, it will create a /dev/pwm10 entry.
Then to issue commands you can use any program that can do file I/O. cat and echo will work.
root@overo# cat /dev/pwm10
PWM10 Frequency 1024 Hz Stopped
root@overo# echo 50 > /dev/pwm10
root@overo:~# cat /dev/pwm10
PWM10 Frequency 1024 Hz Duty Cycle 50%
root@overo:~# echo 80 > /dev/pwm10
root@overo:~# cat /dev/pwm10
PWM10 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.
You have to unload and reload the module to change the frequency.
root@overo:~# rmmod pwm
root@overo:~# insmod pwm.ko frequency=2048
root@overo:~# cat /dev/pwm10
PWM10 Frequency 2048 Hz Stopped
The driver takes care of muxing the output pin correctly and restores the original muxing when it unloads. The default muxing by Gumstix for the PWM pins is to be GPIO.
For now, if you want to change which timer is being used, look at pwm_init().
BEAGLEBOARD Note: 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_DEBUG_POWERDOMAIN is not set
# CONFIG_OMAP_DEBUG_CLOCKDOMAIN is not set
-CONFIG_OMAP_RESET_CLOCKS=y
+# CONFIG_OMAP_RESET_CLOCKS is not set
# CONFIG_OMAP_MUX is not set
CONFIG_OMAP_MCBSP=y
CONFIG_OMAP_MBOX_FWK=m