Re: [Xenomai] Using hardware PWM generators with Xenomai

[Robert <robert-km@o2.pl>]

I have decided to push it here, since it was private message:

Dnia 10 sierpnia 2013 11:44 Sagar Behere <sagar.behere@gmail.com> napisał(a):

> On 08/09/2013 07:28 PM, Robert wrote:
> > I wrote this driver recently, and it generates stable pwm signal. Probably you will have to adapt it to beaglebone black.
> > It is written for Pandaboard and its hardware general purpose timer.
> > There are some ioctl interfaces for user space.
> >
> > https://github.com/rkmiec/pandaboard-pwm-rtdm/tree/dc_engine
> >
> > Do NOT use master branch now.
> 
> Hello Robert,
> 
> Quick question about your pandaboard-pwm-rtdm.
> 
> I see in pwm.c (dc_engine branch), lines 125 and 127, that you use 
> gpio_set_value() to set the level of the gpio pin. This function comes 
> from #include <linux/gpio.h>, which is a standard linux driver, I guess. 
> So every time you call this function, you are making a linux kernel 
> call.. won't that trigger a transition away from xenomai's primary 
> domain? i.e., you task won't be hard realtime anymore..
> 
> I haven't understood xenomai completely, so it is possible that my 
> reasoning is wrong. Could you clarify?
> 
> Thanks and regards,
> Sagar
> 



I'm learning it too.
gpio_set_value() could be called from any context (comments in
drivers/gpio/gpiolib.c), so it shouldn't sleep.
On osciloscope this signal looks very stable. Also servo connected to this
signal is not shaking.


And why I'm toggling gpio:
On pandaboard, hardware outputs from those timers are connected to HDMI
outuput, camera, display (for backlight) etc.
None of them is available on expansion connector. That is why I'm toggling
gpios.

My program differs a little from Andrey.
I'm using hardware interrupts from timers to change value on gpio pin and
he was using rtdm_sleep*() (as far as I remember).