[time-nuts] Firmware WNRO fix for S300/S350 and XLi

[Alex Forencich]

As many folks on this list are probably aware, the GPS week number 
rollover (WNRO) periodically causes older GPS receivers to misbehave.  
One solution is to replace the problematic GPS module with a newer one.  
But many of these older modules will still work, they just need a bit of 
help, in the form of providing a hint about the current year.

The Symmetricom SyncServer S300/S350 and the older XLi units both use an 
HCS12 microcontroller to interface with the GPS module. This 
microcontroller is in the perfect position to send the "set time" 
command to the GPS module during start-up.  It's unfortunate that 
Symmetricom didn't implement something like this in the firmware, for 
instance to pass the time to the GPS receiver when the time is manually 
set on the unit.  Fortunately, not only are these microcontrollers not 
code-protected, but the firmware for the HCS12 microcontroller on the 
S300/S350 is actually included on the CF card so it can be installed 
during an upgrade.  So, with a bit of work in Ghidra, I managed to sort 
out how to patch the firmware to set the initial date on the GPS 
receiver.  With the patch applied, it should be possible to get some 
more life out of GPS receivers that would otherwise not be usable due to 
WNRO, including the Furuno GT-8031F modules as well as iLotus M12M 
modules with older firmware.  Symmetricom kindly put BDM headers on 
their boards, so all that's needed to install the modified firmware is a 
USBDM adapter, which runs for around $30.

It may also be possible to build similar patches for other units, such 
as the S200/S250.  But currently I only have access to S350 and XLi.

Unfortunately, cannot distribute the modified firmware for copyright 
reasons, and I don't yet have a comprehensive write-up of how to apply 
the patch yourself.  It's also not particularly well-tested as I just 
got it working last night.  In the meantime, if anyone is interested in 
these patches, please contact me directly.

-- 
Alex Forencich