On Sat, Feb 5, 2011 at 9:39 AM, Gilles Chanteperdrix <
[email protected]> wrote:
> at91_enthus wrote:
> >>> Rookie mistake. Indeed, I can have everything in userspace using
> Xenomai.
> >> I
>
> Or in kernel-space... with Xenomai RTDM skin.
>
> >>> wrote a simple program that did a periodic job every 100 us and the
> >> jitter
> >>> was very small (around 20 us without load and around 35 us with SSH-ing
> >> and
> >>> Xenomai compilation running on board).
>
> Well, as we often say, benchmarking requires making more than that to
> load the board, and for a long time.
>
> >>>
> >>> Somewhat unrelated to this thread, I found that when I invoked
> >>> rt_task_set_periodic(), I got very precise timings (as expected).
> >> However,
> >>> when I tried to insert small delays using rt_timer_spin(), the errors
> >>> (t_meas - t_spin_func_argument) were not large, but noticeable. For
> >> example,
> >>> I can precisely measure a 50 us periodic task on the scope. With the
> same
> >>> interval in rt_timer_spin(), I get 55-60 us. As far as I understood,
> all
> >>> timing-related functions use the same time base, so what's the reason
> for
> >>> this behavior?
> >>
>
> Well, rt_task_set_periodic gets the hardware timer to tick exactly every
> 50us. So, whatever time it takes between the return of the call to
> rt_task_wait_period() to the entry of the next is irrelevant. And
> particularily, the jitter will not matter as long as it is smaller than
> 50u.
>
> On the other hand, doing:
>
> for(;;) {
> /* Do something */
> rt_timer_spin(50000);
> }
>
> The period of the loop will be 50us + rt_timer_spin jitter + time to "do
> something". And so, will not even be constant.
> You are not creating a periodic task. Besides, the system will not
> execute anything else in user-space than your real-time task, and in
> particular, the Linux will never run. Except of course if "do something"
> contains a call to some secondary domain function, in which case, your
> real-time task no longer is real-time.
>
> So, that is not the correct way to get a periodic task.
>
> --
> Gilles.
>
Actually, the rt_timer_spin was included in this function that is run as a
real time task:
void spin(){
rt_task_set_periodic(&spin_task,TM_NOW, 100000);
while(1){
*((unsigned int *) (piob_base + (PIO_SODR))) = 1<<0;
rt_task_wait_period(NULL);
*((unsigned int *) (piob_base + (PIO_CODR))) = 1<<0;
rt_timer_spin(50000);
}
I decided to go for simple GPIOs manipulation, so I can have a better
picture of what's happening on the scope.
R.
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