'Few hundred mSec', covers a lot of ground. It is sent 'within a few hundred mSec' of the last PPS'. The ZDA message sends the time of the _last_ PPS update. Sorry, my phrasing didn't make it clear what happens. The problem, is that yo DONT know if the UTC time provided at this moment is correct, because SOMETIMES, a few minutes later, the GPS corrects the UTC time, like this: Whit NO PPS signal present on the PPS output, but once you have the PPS signal on the PPS output, the NMEA output will SYNC whit tha falling edge of the PPS signal, like this: If you look at the nmea sentence output from a GPS you will see for example this: Hi asmboy, i understand what you are sugesting. with check function - admittedly using a buncha clocks It actually repeat a zda sentence xxxxxxx.000 The thing is im not "asking" anything, I discovered the problem by listening the nmea output from the GPS. The way to avoid this is to repeatedly read the time, and only accept it at the moment when a 'new' reading appears. The time by it, is now xxxxx+1.00, so you receive from the second unit xxxxx+1.īecause the two clocks are not exactly synchronised, and the read itself takes time, you can always get the possibility of a one second difference. Then one second later _by your clock_, you ask again. Suppose you happen to ask at exactly xxxxx.00 seconds. Asking for it from one unit, then another. You request the data (say) at (nominal) 1 second intervals. The other thing may simply be granularity. I think the clocks are off if the receiver doesn't have a good satellite fix. I know it's available on the GPRMC sentence and I suspect it's also available on others. So, if two consecutive nmea sentences has the same hour, then the GPS has corrected itself. So, no error there, the GPS sends the nmea strings one every second. Jerry I, i don't display the time, i just "spy" the nmea sentences. If i get the hour only one time and then only synchronize it with the PPS, then i could be making a mistake if one or a few boards get the "wrong" UTC time that the GPS sends for the first minutes. I need to get the exact same hour on a few minutes and keep it for about 8 hours. But the users will use the board only a few hours. The thing is that i have to get the SAME exact hour on all of the boards.Įvery board has an RTC which synchronizes every few minutes. Every 24 hours re-sync the RTC chip if required. It may stray 10 sec a month.Īs for your problem are you sure a second has passed before you display the GPS time.Īs suggested, use the first NMEA sentence to set the RTC and keep a "local" (to the PIC) copy of the time which then updates with every PPS rising edge from the receiver. You can re-synchronize the RTC every hour if you like but should not be required. Why don't you just read the GPS Time and set a RTC like DS1307 then display the time from the RTC. the GPS corrects the UTC time and everything is ok. if the hour is not correct it repeat a sentence and correct the hour. so i wait for the boards to have 1.x DOP level. WHATTTTT!!!? it repeats the last UTC time!Īnd from this point the boards have the same time. UTC TIME VS GPS TIME SERIALSo I started SPYING the serial communication and the UTC times, and discovered that sometimes, the NMEA sentences do this after some time: then i simulate the GPS NMEA sentences and no error. The problem is that SOMETIMES the gps have a 1 second difference with each other! The thing is that when they get the position fix it starts to provide the GPZDA sentence and the PPS signal, so, i take the UTC time and PPS and establish the local clock for the board. So i use the nmea sentence "GPZDA" and the PPS signal provided by the GPS. I'm developing a GPS time synchronized clocks, so, i have some boards with a MAESTRO WIRELESS A2235H gps and i want them to have the same exact hour. So if you have some clue about whats happening i will be veeeery happy. Hi! I post this question here because I'm really lost and i know some of you have worked with gps modules before. Please do not post bug Reports on this forum. FAQ Forum Help Official CCS Support Search Register Profile Log in to check your private messages Log inĬCS does not monitor this forum on a regular basis.
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