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A project log for GPS Clock

A simple desk clock that gets extremely accurate time from GPS

Nick SayerNick Sayer 11/27/2021 at 22:320 Comments

Well, between the pandemic and the fire at the Japanese oscillator factory, SkyTraq has told me that they're going to have to EOL the Venus838LPx-T module that I've been using in damn near everything. I've got about 120 of them left, so there's plenty of time, but I have also gotten ahold of their replacement, the PX1100T, which looks to be a whole lot more convenient all around.

Firstly, it's the same price at Q:100. It supplies antenna power on its own, so there's no need for the external on-board bias-T. TBD is it's behavior when faced with a short circuit on the antenna jack. We'll see. It would suck, but in principle we could add a blocking cap and do our own external bias-T with our proven circuit. But even better is that the module is compatible with all four of the existing GNSS systems today - GPS, GLONASS, Galileo and Beidou. It also comes as a castellated PCB module, so no more LGA, which was a fiddly assembly process for me. It has fewer connections, and fewer support components on the board. The one big disadvantage is that there is no FIX LED output pin.

I could get rid of it. For the GPS Clock it's a bit redundant, but for the GPSDOs it's somewhat handy to have it, so I've decided to add support for emulating it on a currently unused pin of the XMega.

This isn't too hard to do - just hit the OUTTGL register's bit for the pin in question in the PPS ISR as long as the GPS Lock indication is set. And any place where you clear the lock indication, hit the OUTSET register on the same pin. That yields the same behavior as before - on for bad GPS, flashing at 1/2 Hz for good.

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