RE: nervous about command timing

From: Michael Thomas <Michael.Thomas_at_dynetics.com> · Date: Mon, 4 Oct 2004 12:54:20 -0500

Sounds like a slick robot app!  But it might need some tweaking to fit my
purpose.  First, I need almost 15kHz.  I was hoping it could get that fast
because the data is handed off to other hardware for processing.  (How much
processing is performed inside the loop that gets 1kHz?)
Second, I need to programmatically know about each period to keep my I/O
coordinated - hence the question about interrupts.  Would this template keep
my inputs and outputs interleaved 1-to-1?

// with comedi commands running
While (!stop_flag){
   i=0;
   while (i<AI_SIZE){     // update (overwrite) each ai sample on dpram
      i += read(comedi_fileno(daq), *(dp_ram_ai+i), AI_SIZE-i);
   }
         // ao from 1-scan-old ai ready, write it
         // (desperately wishing my ao board supported TRIG_FOLLOW)
   write(comedi_fileno(ao), *dp_ram_ao, AO_SIZE);
}

Also, as an engineer who "also writes software", I do middle-school C, not
kernel hacking.  KNOPPIX 3.4 has a 2.6.6 boot option (I've been using
2.4.26), and I heard 2.6 supports preemption, but how do I write a program
using it?  I'm on a dual processor machine, so could I safely lock one
processor indefinitely?  (RTAI would be *way* over my head - I'll probably
have enough trouble learning to make this a kernel module.  I assume I'll
need to, given the timing... one thing at a time!)

Michael Thomas

-----Original Message-----
From: comedi-admin_at_comedi.org [mailto:comedi-admin_at_comedi.org] On Behalf Of
Bernd Porr
Sent: Monday, October 04, 2004 8:56 AM
To: Nils Juergens
Cc: comedi_at_comedi.org; Tao Geng
Subject: Re: nervous about command timing

Hi!

I do this usually with two comedi commands. One to aquire the data and 
one which sends out the stuff. Under 2.6 with the preemptive option I 
get 1kHz realtime I/O with just a "while (1)"-loop. My colleague Tao is 
controlling a walking robot in that way. I've copied the mail to him.

/Bernd

www:    http://www.berndporr.me.uk/
         http://www.linux-usb-daq.co.uk/
Mobile: +44 (0)7840 340069
Work:   +44 (0)141 330 5237
         University of Glasgow
         Department of Electronics & Electrical Engineering
         Room 519, Rankine Building, Oakfield Avenue,
         Glasgow, G12 8LT

Nils Juergens wrote:
> On Fri, 01.10.04, Michael Thomas <Michael.Thomas_at_dynetics.com> wrote:
> 
> 
>>trigger?  That still wouldn't help my user code stay synced up.  It just
>>seems there should be a better way.
> 
> 
> If it's a real time problem (it smells like one :-)) ) you need a real
time
> system, e.g. RTAI.
> 
> It might work like this:
> 
> * create an RTAI task
> * make task periodic
> 
> in the task (via kcomedilib)
> * read samples (comedi_do_insn())
> * do stuff (calculations etc.)
> * write samples (comedi_do_insn())
> 
> I think it's also possible to combine this method and comedi_command (for
> increased performance and decreased jitter in the sampling periode).
> 
> hth,
> Nils
> 

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