Hi Adrian,
Then I apologize for doing such a poor job of getting my meaning across. What
I was trying to show was that for a number of reasons, you can't necessarily
trust the trailing edge of a 1PPS pulse. If you're generating a pulse every
second from a clock, and that's all your doing, then, yes, it's an easy job to
make the pulsewidth very stable at whatever value you choose.
Bob
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From: Adrian Godwin <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Cc: Tom Van Baak <[email protected]>
Sent: Wednesday, November 9, 2016 5:39 PM
Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC
serial port for ntpd
But trailing isn't the same as falling. The leading edge can be of either
polarity.
On Wed, Nov 9, 2016 at 11:28 PM, Bob Stewart <[email protected]> wrote:
Tom,
How many times have people posted here, on time-nuts, not to trust the trailing
edge of a 1PPS pulse?
Bob -------------------------- ------------------------------ ---------
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/ GFS-GPSDOs/info
From: Tom Van Baak <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>
Sent: Wednesday, November 9, 2016 4:44 PM
Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC
serial port for ntpd
Bob,
The PIC's I use have essentially no jitter. If they generate a 1PPS the edge
and the pulse width are perfect, down to picoseconds. The talk about "other
stuff" and "priority" and "number of compares" and "ambiguity" is worrisome. It
sounds like a design or coding flaw to me, like what happens when people try to
do precise time with a high level language.
/tvb
----- Original Message -----
From: "Bob Stewart" <[email protected]>
To: "Discussion of precise time and frequency measurement" <[email protected]>
Sent: Wednesday, November 09, 2016 9:30 AM
Subject: Re: [time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC
serial port for ntpd
I'd like to comment on the idea of measuring the width of the pulse. My
experience with creating the 1PPS from an interrupt is that it's fairly
straightforward how to do the set interrupt: The interrupt happens, you execute
one, maybe two instructions to raise the output pin, and you leave the
interrupt routine. But, resetting the pin is a different story. Unless you've
got a lot of interrupt vectors to play with, the reset part of the 1PPS signal
is delegated to a general purpose timing interrupt where you're doing a lot of
other stuff - and it has a somewhat lower priority. So, you wind up doing a
number of compares to see if you should reset the pin, which adds some
ambiguity to the set/reset times.
Granted, I'm doing this on a general purpose PIC, but I have read comments
about various receivers having some jitter on the reset side of their 1PPS
pulse.
Bob
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