USB GPS module

To use, just connect an antenna (either via the u.FL connector or with a pigtail) to the u.FL jack. When powered up, the module will supply 3.3v of active antenna power (this is short-circuit protected if you'r'e using a passive antenna that is a DC short), then connect to a host via the USB-C receptacle.

The host will see a CDC device. Open it and set the baud rate to 115200 baud and the format to 8N1. You'll immediately see NMEA sentences. When/if a good fix is available, PPS pulses will show on DCD. If you want them on DSR or CTS instead, then you can solder closed the relevant jumpers on the board.

I'm having a little trouble with Linux.

As I described in the project details, I've connected the PPS signal to DCD (via an inverter) so that it can be sensed over USB by the host. I've been using the ppscheck utility to attempt to verify that this is working, but it isn't. If I short DCD and DSR together, ppscheck does show the transitions on DSR and they are timed correctly, so I know it should be working in principle at least.

The trouble with using DSR is that the Linux PPS line discipline expects you to use DCD.

So going forward, I've decided to add solder jumpers to the board to allow you to select any of DCD, DSR or CTS to receive the PPS signal. The default will be DCD, but it'll be changeable just by adding or removing solder to one or more of the jumpers. I've also added serial RX and TX LEDs to the board for troubleshooting.

The first prototype has begun testing. The first thing that was discovered was that the BSS84 is not a good choice for the power gating transistor at 3.3 volts. The power output on the drain was closer to 2 volts, and was insufficient to allow the module to power up. Replacing the transistor with a wire from source to drain fixed the problem, albeit without being compliant with the USB spec.

The best case scenario is simply swapping out for a better transistor solving the problem. At the moment, the candidate is the SI2329DS-T1-GE3. It has a lower Vgs to Rds-on resistance curve, making it more likely to pass sufficient current when Vgs is only 3.3 volts.

EDIT: The FDV304P is also a good choice, and I’ve confirmed it works.