In my various tinkering it always seems that I have to recreate a software clock that accounts for the behavior thats advanced by 1 second. Have done that on 2-3 projects the most recent being the GOES DC468 simulator. Regards
On Thu, Dec 23, 2010 at 5:49 AM, Kiwi Geoff <[email protected]> wrote: > Hal Murray wrote: > > I assume it was working correctly previously. If not... At least one > GPS > > receiver I've used is off by a second. There is an ambiguity as to > whether > > the time applies to the previous or next PPS. > > You were correct Hal, the problem with the new firmware for the 18x, > is the time relationship between the 1PPS transition and the serial > data. > > My application required the RMC and GGA sentence at 4800 BAUD. > > So I measured the start and end times of the serial data (with respect > the 1PPS). > > Using version 3.00 firmware on the Garmin 18x, the NMEA data starts > 450 ms after the 1PPS transition, and finishes at 740 ms. > > However using the latest firmware version (3.50) for the 18x, the NMEA > data starts at 750 ms and finishes 39 ms into the "following" second. > That is why my software was loading the wrong time. > > These start and end times of the NMEA serial data change quite a bit as the > workload of the GPS changes. > > There is no mention in all the update notes, that Garmin have changed > the timing of the NMEA data with respect the 1PPS transition. > > Running the same test with my GPS 18, the NMEA starts about 50 ms and > finishes 350 ms after the 1PPS transition - nice and reliable! > > I think the reason my 18x started (in the last few days) to show the > wrong time, is that the trees around my house are at the peak of > growing leaves (Summer in the Southern Hemisphere) and so this would > have caused the GPS to work harder, and so must have delayed the NMEA > data enough (with version 3.50) to give the wrong time on the serial > data. > > So the moral of the story, don't expect that a device will be "better" > if you upgrade to the latest version of software. > > Thanks Hal for your kind pointer - I now understand what is going on, > and not to be so excited about new firmware updates. > > Kiwi Geoff (Christchurch, New Zealand). > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
