Teensy 3.2 Not properly working when using trimpots with an external non-alimentation

[flyG]

Not sure if I made an obvious mistake but I'm using a custom PCB made for the OctoWS2811 library.

On this PCB I have 3 trimpots wired to the 3.3V output of the teensy:



Pin A3, A6, A9 that I intended to use as INPUT.

I've got a pretty basic alimentation that can turn on the teensy via VIN (5V regulator outputing between 4,6 and 5V) with a LM2937ET-5.0/NOPB.



When I use the USB to power on the teensy (vin switch off), everything is working correctly.

When I use only the regulated 5V VIN (no USB) the teensy is most of the time not working and it looks like the position of the trimpots can turn it on or off.

I've got a FastLED w/ OctoWS2811 sketch running.

One strange thing is that the onboard LED (13) is somehow changing intensity when I turn the trimpot (it's supposed to be blinking fast, works via USB), while another LED paired with a 100R it blinking correctly on the same PIN.

I don't understand what's happening as to my understanding it should work the same as long as the teensy is properly powered (and 4.6V should be enough).

I guess it's a hardware issue and that I'll have to update the layout/wiring.

Will gladly give more info if required. Thanks!

EDIT: can someone update the title, forgot the USB part (non-USB alimentation)!

 

[Theremingenieur]

The schematic looks ok, so this might be a "reality vs theory" issue:
a) Are you sure about your wiring?
b) It might be that the input power before the voltage regulator has a too high inner resistance and goes temporarily down. In that case, a multimeter isn't helpful since it always shows an average, check the input and the output of the LM2937 with an oscilloscope!

 

[GremlinWrangler]

What may also be happening here is that you don't actually have the Teensy powered properly from it's external supply (but USB is) in this case the protection diodes may allow enough current through the pots and the diodes to make the chip at least sorta work.

Where is the 3.3V for the pots coming from, and is it pots high or pots low that allow things to work? Some voltage measurments with respect to gnd on your 5V supply may be informative (look for voltages that should be the same that show a drop).