How to disable the single blue (1PPS) LED

[ajorg]

There is an explanation at meshtastic/firmware#1493 (comment) that seems to suggest that this blue LED cannot be disabled (not the blue LED from the tri-color LED, but the blue LED that's connected to the GPS 1PPS). But it's unclear if the meaning is that the LED cannot be disabled while the GPS is enabled, or if correctly disabling the GPS module should also disable the LED.

If disabling the GPS (or 1PPS) module correctly should disable its blue LED, clarification is needed as to the correct method of disabling the module.

If it's not possible, we'd appreciate positive confirmation that this blue LED cannot be disabled under any circumstance.

[nxxxse]

I am also affected by this on my T-Echo with Mestastic 2.1.18. I continued where @richteel left of in meshtastic/firmware#1493 (comment). I checked with a Multimeter.

Is the L76K really in standby?

Yes. https://github.com/meshtastic/firmware/blob/c75965480f9da9f067cf0d4722c06bf31ca7f959/src/gps/GPS.cpp#L629 works and pulls the WAKEUP pin on L76K to GND. I measured 0.0V. Meshtastic is off/in deep sleep and the internal blue LED is on. I measure 2.5 V on the 1PPS pin. I would have expected 3.3 V because that is VCC of L76K even in deep sleep of nrf52840.

RESET_N is 0 V … Interesting. Theory is that the GNSS module constantly resets and 1PPS is LOW in that state? I tested by putting:

digitalWrite(PIN_GPS_REINIT, 1);
pinMode(PIN_GPS_REINIT, OUTPUT);

into GPS::sleep(). I confirmed that RESET_N is now 3.3 V with that patch in T-Echo deep sleep but still, the LED is on.

What is interesting is that when T-Echo is on and I probe GND and RESET_N in voltage measuring mode, the internal blue LED comes on whenever I touch both pins. RESET_N seems quite sensitive and the high resistance of my multimeter is enough to trigger it. I measure 3.3 V so it is not like the ~1 MΩ of my multimeter would pull the pin down to GND. I don‘t have a oscilloscope here to look into this further.

If it is, why is 1PPS held high?
Is 1PPS really active low and should have the LED polarity flipped and connected to VCC rather than GND?
Is it best to simply remove LED6? (Most likely this is the correct answer but I do not like to jump to conclusions. The previous questions need to be answered first.)

I desoldered the internal LED as a quick fix to not drain the batteries again. It was discharged to 3.0 V over a few months not not being used. Not sure why it was discharged that deep when UMW TP4054 cuts of between 3.4 to 3.8 V with a hysteresis of ~200 mV.

[FabianMNoss]

@nxxxse I have a scope here, but had no time to go into the details of this bug yet. If you are interested, let's try together to sort this out?