Looking for help on a truely bizzare Rx problem

[mackrackit]

Just a thought...

How close is the GPS unit to the MCU or OSC when they are on the same board? It is pretty easy to "jam" the GPS signal. I will assume the GPS unit has a built in antenna?

[khufumen]

The GPS is a couple inches away from the Pic and its oscillator. However I know the GPS tx stream is working because I can read it on the test board and anything I send from the test board to the PIC on the main pcb board is easily read. Really weird. I'm going to guess that there is some interference and that it's back to the drawing board and another donation to PCBExpress.

[languer]

Maybe you already decided the path forward, but let's try to summarize what you have.

• GPS on its own test board (call it external GPS)
• 18F4525 on its own test board (call it external MCU)
• GPS and 18F4525 on shared PCB (call it internal GPS and internal MCU)

• Both GPS modules are identical
• Both 18F4525 have same code

• Internal GPS does no communicate with internal 18F4525 (on shared PCB)nd internal MCU?)
• Internal GPS does communicate with external 18F4525 - again, look at the connections/traces on the PCB (i.e. did you not somehow crossed the internal traces?)
• External GPS does communicate with internal 18F4525 - are the connections identical between the internal GPS and external GPS (i.e. did you not somehow crossed the internal traces between internal GPS a
• External GPS does communicate with internal 18F4525 only when power is not shared - are you sharing bypass caps between internal GPS and internal MCU? Is internal MCU operating at all - could you have brownout or reset issues?)

How are you connecting the internal GPS to the external MCU? Are they sharing the same supply, is the internal MCU still connected, is the internal MCU running?

I doubt any of these will solve your problem, but would be simple questions to hopefully "steer" you in the right direction. I would think that if the GPS is working, even if the signal is "jammed", it would still have an output.