[time-nuts] My Garmin 18x, Ver 3.50, currently 1 second slow to UTC

Kiwi Geoff geoff36 at gmail.com
Wed Jan 5 11:08:38 UTC 2011

Hal Murray wrote:
> It would be really neat if you could run similar experiments on 3.3 and/or
> 3.5.

I have run some more tests Hal, with V3.5, averaging ON and OFF.

I include V3.2 for comparison.

I measured the time of the "START" character ($) of the $GPRMC
sentence and  the "LAST" 0x10 (Linefeed) character, relative to the
leading transition of the 1PPS, GPS 18x set to 4800 Baud.

Version 3.20 firmware for Garmin 18x
START Min =  370
START Avg =  433  (N = 7200)
START Max =  518
LAST Min =  519
LAST Avg =  582  (N = 7200)
LAST Max =  667

Version 3.50 firmware for Garmin 18x  with Averaging ON
START Min =  768
START Avg =  813  (N = 4194)
START Max =  871
LAST Min =   917
LAST Avg =   962  (N = 4194)
LAST Max =   1020  ( 7 sentences finished over 1,000 ms)

Version 3.50 firmware for Garmin 18x  with Averaging OFF
START Min =  769
START Avg =  805  (N = 3563)
START Max =  858
LAST Min =   918
LAST Avg =   954  (N = 3563)
LAST Max =   1007 ( 4 sentences finished over 1,000 ms)

This agrees with what Kasper Pedersen wrote earlier:
> 3.10 starts transmitting 350ms .. 480ms after PPS
> 3.30 starts transmitting 920ms .. 1220ms (!) after PPS
> 3.50 starts transmitting 700ms .. 1220ms (!) after PPS

The "averaging" appears to have about an 8 ms penalty in delaying the
NMEA data stream.

We can see in the above, that V3.5 RMC is infrequently not finishing
"within" the correct second !

For "my" application I need both RMC and GGA to finish within the
current second - which the Garmin 18x does very comfortably with
version 3.2 and under.

Kevin Rosenberg wrote:
> Has anyone had trouble with reverting firmware upgrades on
> Garmin products? Of course, feedback on going back from
> 3.50 to 3.20 on an 18x would be most specific to my situation.

I have "reverted" quite a number of times now Kevin - trying to
understand why the 18x can be 1 second slow. I have never had an issue
putting in an older version into the 18x, the updater software does
know what your doing, it says:

"Your current software version is newer than the update version.

Do you want to replace your current software version with an older version ? "

We click the YES box.

To do the above timing tests I had v3.5 loaded into my 18x, I then
reverted to v3.2 and everything worked fine Kevin (to answer your
specific question).

Regards, Kiwi Geoff (New Zealand).

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