[ntp:hackers] Problems with Oncore???

Terje Mathisen terje.mathisen at hda.hydro.com
Wed Jun 1 03:58:18 PDT 2005


bg at lysator.liu.se wrote:
> I have a small box converting TTL <-> RS232 with a standard
> MAX232. This conversion adds ca 1.5us delay. 

OK, that's in the proper ballpark, and similar to what I expect from my
old TAPR interface board.
> 
> However I between my old Jupiter and a Trimble Acutime2000 (palisade)
> I also see a difference of 13us, like Terje have with different
> equipment. I have not fully explored which one is of by 13us in my system.

See my previous reply to Poul-Henning: This is what you get when you
have PPS signals on two different serial ports on the same PC!

Both signals occur effectively at the same time, or at least closer
together than the hw interrupt latency on the PC.

This means that the first (probably COM2 which is at IRQ 3) serial port
to be handled will timestamp the interrupt, and only then can it go on
to the other (COM1 at IRQ 4) signal. This gives a consisten bias for the
 lower priority port.

Oncore has a perfect fix for this, in that the actual time of the PPS
signal can be offset an arbitrary amount from the real top of the second.

By setting it way off, at 1 ms, the two tasks won't ever interfere, and
I get pretty much identical performance from both sources. :-)

Terje

-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"



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