Re: [Xenomai] Using hardware PWM generators with Xenomai

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 08/08/2013 03:36 PM, Sagar Behere wrote:
> Hello,
> 
> I wish to generate PWM signals from Xenomai, using the beaglebone black, 
> kernel 3.8.13 patched with xenomai.
> 
> There already exist linux kernel modules for the hardware PWM generator 
> (eHRPWM) on the am335x chip in the beaglebone. The PWM generator can be 
> configured and controlled via the /sysfs interface and the whole thing 
> works very well.
> 
> I understand that the /sysfs interface cannot be used by xenomai tasks 
> without triggering a transition away from the primary xenomai (hard 
> realtime) domain. So my question is: What is the least effort way to 
> change the duty cycle of the hardware PWM generator, from a xenomai task?
> 
> Does the following approach sound feasible?
> 
> 1. Configure the PWM generator (freq, polarity etc.) from the /sysfs 
> interface at application startup. This need not be realtime
> 2. Assuming that the duty-cycle is controlled by the value of some 
> memory-mapped register, use mmap()/ioremap() to map that register's 
> address into the xenomai task's address space.
> 3. Write the duty-cycle values to the mapped memory, from within the 
> xenomai task
> 
> So this is like a hybrid approach that uses the existing linux kernel 
> module for initializing/configuring the hardware PWM and the xenomai 
> task only changes the value of one register that affects the duty cycle 
> of the output waveform.
> 
> Thanks in advance,

It could probably work, but does not seem like a good idea:
- it is not future proof, as you will need to change the application
code if one day you want to run the application with different hardware,
an RTDM driver with a well defined interface would avoid that;
- it is not really safe as it does not prevent several applications to
access the same hardware at the same time.

Regards.

-- 
                                                                Gilles.