The advantage of using a hardware PWM timer (like OMAP3 GPTimer8-11) is
that there is no interrupt load on the system.
(The PWM signals are completely generated by the hardware.)
PWM outputs controlled by interrupts require one interrupt for each edge
transition of the output signal.
The disadvantage of using a hardware PWM timer is that there are only 4
PWM timers on the OMAP3 chip (3 accessible on the BeagleBoard). Once you
have used all 4, than you must use the method Gilles describes.
Regards,
Bob Feretich
On 9/8/2011 2:13 AM, Gilles Chanteperdrix wrote:
On 09/08/2011 10:21 AM, Andrey Nechypurenko wrote:
So I wrote
a RTDM driver to use the three that were pinned out, and used real time
limit and overflow interrupts to to wiggle a GPIO for the fourth PWM output.
It seams to me that you are using different implementation approach
then suggested by Gilles in this thread: "I was talking
about a Xenomai driver, using the RTDM skin using for instance two
timers, a periodic timer which handler would set the gpio to 1 and
launch the second timer, a one shot time which handler would set the
gpio to 0."
If I understand correctly, the difference is that Bob proposes you to
dedicate a hardware timer to the PWM, whereas I propose you to use the
software timers, which ultimately, also depend on a dmtimer. I would not
expect big differences between the two approaches, neither in terms of
jitter, nor in terms of code.
Could you please say a few words about the reasons why you chose this
particular way?
The Linux version of this driver made the 4th servomotor growl. There is no
growl using the RTDM version. I asked a lab technician to probe the four
servo PWM signals and tell me which was software GPIO generated. He could
not tell.
timing. So there might be fundamental difference between periodic task
launched in user-space with rt_task_start/rt_task_set_periodic and
similar periodic task running as RTDM driver which I do not
understand. If this assumption is true, I would really appreciate any
pointers to the documentation which explains what are the differences
and why RTDM driver can deliver better rt performance then Xenomai
thread from user-space.
As suggested in the previous mail I sent you, compare the differences in
latencies with latency -t 2 between user-space task and kernel-space
timer. You will see that a timer handler has much less jitter. Switch to
user-space, and context switch time explain the difference.
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