Re: [Xenomai-help] Jitter while generating PWMs with GPIO from RTDM driver

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

On 04/23/2012 02:35 PM, Andrey Nechypurenko wrote:
>>> retval = rtdm_task_init(&pwm_task[i], // there is currently only one
>>> element in this array
>>>                       "pwm-task",
>>>                       pwm_task_proc,
>>>                       0,
>>>                       RTDM_TASK_HIGHEST_PRIORITY,
>>>                       20000000); // 20ms period
>>
>> Do not use a thread, use a timer.
> 
> So you mean instead of starting periodic task with rtdm_task_init() it
> is better use timer functions to trigger pin toggling piece of code?
> Could you please elaborate on it a little? I thought that
> rtdm_task_sleep() and rtdm_task_wait_period() are using timers
> internally to wake up the thread at the right moment. Is not they?

Yes, but once the timer is woken up, a context switch is needed to wake
up the thread, this adds time.

> 
> Is not it a kind of work-around you suggesting? If there are some
> problems which led to the imprecise timing of the sleep/wait functions
> mentioned above, then, if technically possible, it would be better to
> fix them instead of working around this behavior.

No, the time it takes to switch context between threads is unavoidable.
So, if you want to avoid it, you have to use a timer (rtdm_timer_init),
note that it is really common in RTOS interfaces to offer a timer API,
this is not a workaround at all.

The other alternative I describe in my last mail, that is, using a
dedicated hardware timer with its own irq handler, is a bit more of a
workaround, but still not uncommon in the RTOS world.

-- 
					    Gilles.