I haven't checked the power supply with my multimeter yet, but am curious as to the following:
I have a GPS in a bread board, connected via software serial to an Uno. I have a Sparkfun BB power supply and can power the GPS from that power supply. Rx and Tx beween GPS and Uno are switched, as required.
If I power the GPS by the power supply, at 3V3 or 5V, the readings from the GPS are garbage. If I power the GPS from 3V3 or 5V from the Uno, the readings from the GPS are pristine, and the GPS locks my location, indoors, as verified via a google maps lookup.
When I come to use this GPS in a circuit, it will be powered by the same source as the uC I intend to use, with common ground, so there should be no issues.
What I want to understand is why the external power supply is causing this issue. Is it because the serial signal on the Uno side relies on signal edges from a ground (reference) that is not shared by the GPS unit?
I am not really sure what to search for to find resources to learn, so pointers would be appreciated.