Teensy 4.0 External Power

[JDC762]

I am having issues power the Teensy 4.0 from an external source to the USB ie VUSB.
I have cut the pads to separate Vin from VUSB.
I have then applied 5v to Vin and I get nothing from the Teensy.
Probe the 3.3v pin and there is 3.3v there.
If I remove 5v from Vin, and then apply 3.3v to the 3.3v pin, again the Teensy does not spark up.
Reconnect VUSB to Vin and it all works as expected.

I have read the thread "Teensy 4.0 Can't program when using external power supply"
but there does not seem to be a definitive outcome to that thread; plus it is suggesting that is powering up ok, just not programming.

I have two Teensy 4.0 and they both behave the same.

I have reviewed the schematic, and it should be a simple, cut the pads, and apply 5v to Vin.
However, when you disconnect the USB, then the Teensy starts to work.
My only thought is the GND from the USB is dragging something down, and maybe there needs to be 2 pads one for VUSB to Vin and one for the USB Gnd to Gnd.

 

[manitou]

does the simple blink sketch work on USB and also on just external Vin?
make sure your test sketch does not have while(!Serial); -- that will stall the sketch if not hooked to USB

as the rule at the top of the thread says, post complete source code for your test sketch.

 

[JDC762]

I am using Blink, as this is not a code issue but a hardware one.
When the Vin is powered by VUSB (ie PAD connected) the Teensy works 100% and shows in the IDE monitor.
Cut the pads and connect the positive feed via Vin, and not VUSB, and the Gnd from the external power is connected to board, which in turn is connected to the USB; as per the schematic.
Now the Teensy will not work.

So I made a USB cable without the positive AND negative, so only the data lines.
Now the Teensy works 100%.
Teensy can be powered via Vin at say 5v or can be feed by a 3.3v feed to the the 3.3v pin on the board.

My point is there should be pads to allow for the USB negative to be isolated.
It seems that leaving the USB negative connected, creates a potential divider and so the Teensy does not work.
I know Paul shows this in a post on External Power, but it is not reinforced for all boards.

This is now for information.

 

[defragster]

There is something else going on because this has been many times to work. The GND path through USB cable will be used in some situations - on a T_3.6 once had an RGB matrix here feeding over 1 Amp back through GND because external power GND path offered more resistance until GND path to that external supply was improved.

But if the external supply has a good GND using proper common ground pin and VUSB through hole for power it does work.

You may have found something unique about T_4.0 - but more detail is needed - photos and code perhaps to understand this because what is described and indicated should work.