In my images, the ARM core is running motion and trajectory planning in
a 1 ms (configurable) loop, using the Xenomai real-time patches.  If you
don't have something like Xenomai or PREEMPT_RT patched into the stock
Linux kernel, the 1 mS loops do not execute at the proper time.

The PRUs are used to generate step/direction pulses at (potentially)
high speed, running simple code in a short loop (typically 1-10 uS),
which is not possible on the ARM.  Even with Xenomai, IRQ latency is on
the order of 20-60 uS for the ARM, so it just can't run a task loop that
fast.  The PRUs don't have anything else to do, so they can happily make
step/dir pulses with good timings while the ARM core handles the
complicated not-so-time-critical stuff like motion planning, and the
non-real time stuff like a user interface.

On 3/31/2014 10:33 PM, Brad Hopper wrote:
> Charles any quick points or a link to something explaining what is the 
> meaning of real time linux, especially in the context of having PRUs - I 
> sort of had the sense that, for example in your machine kit image, the real 
> time bits were being performed by the PRUs, so what is the reason for 
> Xenomai? You have to run some OS of course, but if you have a dedicated 
> subsystem that just comes up for air when it needs to inform the OS, why 
> not just ?plain? linux? And if linux can run real time, why do you need the 
> PRUs? maybe it's a matter of the scale of latency we're talking about.
> 
> On Monday, March 31, 2014 6:08:27 PM UTC-4, Charles Steinkuehler wrote:
>>
>> On 3/31/2014 5:54 AM, Eren Basturk wrote: 
>>> Hi All, 
>>>
>>> I wonder that Is there any real time operating system to use on Beagle 
>> Bone 
>>> Black? Could you give me some information about real time linux on 
>> Beagle 
>>> Bone? 
>>
>> TI has a list, but it doesn't include everything: 
>>
>> http://www.ti.com/lsds/ti/tools-software/rtos.page 
>>
>> ...conspicuously absent are the various non-commercial solutions. 
>>
>> Is there something in particular you are wanting to run? 
>>
>> -- 
>> Charles Steinkuehler 
>> cha...@steinkuehler.net <javascript:> 
>>
> 


-- 
Charles Steinkuehler
char...@steinkuehler.net

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