[time-nuts] Re: HP Z3801A project update

[Keelan Lightfoot]

Depending on who’s GPS chipset is being used, a fix might be possible. I’ve poked around inside the firmware of a number of Trimble receivers (so far three generations of the 4000 series, and the Placer series). Because the first week rollover occurred in 1999, any receiver made close to that date already has logic to handle a week number rollover. Finding this in the disassembled code is usually easy because they simply add 1024 to the week number, which usually appears as an add instruction with a 0x400 immediate operand. changing a 0x400 to 0x800 will extend the receiver’s lifetime another 20 years. The most difficult part of this is figuring out the processor used on the receiver, and figuring out how to get the patched firmware back into the receiver.

- Keelan

> On Feb 3, 2022, at 8:50 PM, Bill Beam via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> The date issue is a 1024 week rollover failure in the GPS receiver.
> A fix is to replace the GPS receiver with one that does not have this problem.