Teensy 3.2 power-up problem

[stg]

Hello!

I am having an issue with a Teensy 3.2 that will not properly power up when supplied by external 3V3.
There is not much of any schematic to speak of except the 3V3 regulator and capacitors (3.3V rail is checked to be stable, noise free and without any over-voltage condition).
Also, there is no source code beyond Serial.begin in setup and Serial.println + delay in loop.

If I power the Teensy up using a USB cable it starts up just fine. I can then power up the external 3.3V supply and disconnect the USB - the Teensy will keep running just fine.

However, when I apply the external 3.3V source without having connected USB it does not boot. Connecting USB at this time does nothing and the Teensy does not identify over USB or run the programmed code.
I've found no way to get the teensy to recover from this state, except a reset.
This can be done either by the reset line, program button or simply disconnecting everything and subsequently powering up over USB.

Removing USB support makes no difference.

Any suggestions at all would be highly welcome.

Best regards,
Davey

 

[tonton81]

i do believe you cant feed 3.3v to the VIN pin as the power is not sufficient for the linear regulator. you need to supply 5v to the VIN pin which is why it works for the usb and not your voltage source (3.3v)

 

[KurtE]

Warning, I do not run my Teeensy from External 3.3v supply.
You can find the schematic up at: https://www.pjrc.com/teensy/schematic.html

The VIN pin, states that you can supply it with voltage between 3.6-6v.
So 3.3v is below that.

However you can probably supply 3.3v to one of the 3.3v pins on the Teensy, which bypasses the voltage regulator. However if you then plug in USB you will be powering the VR and the 3.3v, which may or may not be bad.

You could cut the VIN/VUSB jumper on the bottom of the board, which again keeps these separate...

 

[stg]

i do believe you cant feed 3.3v to the VIN pin ...

That is correct. The VIN pin is routed to an LDO which would drop about half a volt.
I am powering the Teensy via the 3V3 rail.

As it turns out the Teensy had been damaged.

Popping a spare onto the board solved the issue and the unit is now running on its own power:



Thank you for your suggestions!

Have a lovely day,
Davey