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.