Re: [gpsd-dev] RFC 2783

[Gary E. Miller]

Yo Hal!

On Tue, 10 Mar 2015 13:54:17 -0700
Hal Murray <address@hidden> wrote:

> >> USB is polled.  ...
> > No, USB 1.1 is scheduled, at least on the PL2303.  1024 slots.  So
> > we see in practice about +/1 600 uSec on the GR-601W. 
> 
> We are saying the same thing, just using different words.

Mostly.

> The key idea is that the polling is using a periodic schedule.  There
> is no interrupt mechanism.  The closest equivalent of interrupt
> latency is polling interval.

Yup.

> > That may seem poor, but is way better than I get with just NTP over
> > my local GigE net. 
> 
> Most GigE drivers batch interrupts to reduce CPU load.  What's your
> round trip time?  Mine is typically 200 microseconds.

I'm 200 to 300 mSec.  With software PTP I can get it down to 6 uSec.

> For the Raspberry Pi, the Ethernet is on USB so there is more fuzz
> there.  I assume it's polled more frequently than 1 ms but I don't
> have numbers handy.

USB 2.0 10/100 Ethernet.  Around 600 to 700 mSec.  USB 2.0 micro frames 
are 125 uSec.  TBD how good a timeing I can get on 2.0.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
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