[time-nuts] BC637PCI 1024 week rollover

[Magnus Danielson]

On 10/02/14 11:15, GandalfG8 at aol.com wrote:
> Ah, I took 1999 as I thought that was the only relevant date for another
> 1024 weeks, I'm not familiar with the shifted 1024 week period so will take a
>   look at that.
>
> Does "shifted" imply a shift at the whim of the manufacturer, ie  could it
> explain why these boards might have been ok a few years  ago but not now?

Yes. We have seen week 500 and week 512 occuring.

Considering this simple code:

if (gpsweek < 500)
	gpsweek += 1024;

This means that GPS week 500 to 1023 maps straight and truncated GPS 
week 0 to 499 is mapped to GPS week 1024 to 1523.

However, when GPS week 1524 occurs, GPS week 500 is transmitted, so 
receivers jump from GPS week 1523 to GPS week 500 and the NMEA readout 
date jumps 19.3 years. Woops.

The interesting thing is that the GPS otherwise operate properly, as it 
is only the read-out date which goes wrong, not the internal gears of 
the GPS, so the leap second applied will be the current and not the one 
from 19 years ago.

> Oh dear, I think a wee light bulb has just exploded:-)

Good. :)

> I haven't checked this yet, but if shifting means to  start a 1024 week
> period that's approximately from or not too  far before the date of
> manufacture, either for individual units or just as  a ballpark for a given production
> run, that would buy them nearly twenty  years from then, which would mean
> these boards should still be ok.

It's arbitrary. It could be from writing the code to just before a 
certain batch. Who knows. Adjusting it is trivial.

> If shifting means to do this say at the design stage or starting with the
> first production run then they might buy twenty years from then but
> regardless of individual manufacturing date.

It's arbitrary. Considering that GPS week 500 and GPS week 512 have been 
found in equipment, and these are not "random numbers", it seems like a 
random pick early in the design.

> I'm not too sure that even the earliest of these boards should be twenty
> years old yet, but if plan Z was to stick with some previously picked
> arbitrary  date, such as company formation or granny's birthday, then that might
> well be  the answer:-)
>
> Thank you, will definitely look more closely at this, perhaps it's not time
>   yet to put the boards back into hibernation after all:-)

Good, now you learned something. :)

> I agree re the TMS29F010, and I'm sure I could read it, but would
> definitely need an adapter for that.

Ah. Yes.

I don't know what FW my boards have, if it has the GPS FW latent or not.

Cheers,
Magnus