2 Teensy 4.1s with no 3V or USB connection

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david.skovron

New member
I have a tricky one here. I have (2) Teensy 4.1s on my desk with an uncertain past. I am not totally sure how they were used but they supposedly are spares that have been living in a tech closet and never actually deployed. They also supposedly worked fine until I started testing just now. These were installed in a custom board that receives 12VDC, we have many Teensys running on these boards throughout our space and have not had any issues. For my testing I removed the units from these boards and attempted to power solely through USB (VIN and VUSB trace wire was NOT cut). In fact I wonder if someone connected USB while connected to external power (would that fry the board?)

Here are the symptoms:

-5V reads fine when powered with microusb
-No usb connection: cannot see unit on Arduino IDE, Teensy loader also does nothing when attempting to load blink, tested on Windows and Mac and multiple cables
-(1) unit reads 0.03V on the 3V pin, the other unit reads 0.66V
-Factory reset had no effect.

I am NOT a computer engineer and am relatively new to Teensys. We use many of these units throughout our space, so finding the failure is more to prevent future issues and learn a bit more than anything. That being said, I also do not have intricate soldering tools and not a whole lot of time to spend on troubleshooting. I'm already wondering if the best bet is to send these units back to PJRC.

I know these are not ideal circumstances with little info, but any help is appreciated.
 
Yes, connecting USB while powered without VUSB cut would damage it. Sounds like it may be case if your getting that low voltage on the 3V pin.
 
david.skovron said:
If that were the case is there any way to resurrect these units, or should I just start ordering my replacements?
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It might just be the 3.3V regulator, but there pretty much isn't a way to replace that easily. I would probably replace those. Have you tried powering it with 3.3v trough vin without the usb connected, and does the board get hot when powered?
 
david.skovron said:
I don't have a voltage adjustable power supply lying around but I'll see if I can buy/borrow one. The board gets slightly warm but I would not call it hot.
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It might be more trouble than it's worth since you don't have one, if its not connecting to usb it's probably better just to replace them.
 
Maybe the Power Up Sequence info on the T4 bootloader chip page can help with understanding the circuitry as you try to diagnose the problem.

PJRC Store

www.pjrc.com www.pjrc.com

On the board showing nearly zero volts, try measuring PMIC_ON_REQ (see the test points diagram on the Teensy 4.1 page) to see if the RT1062 is requesting the 3.3V power to turn on. If it is at least 1.0V, then look for whatever is shorting out the 3.3V power.

Usually when something is shorting out the 3.3V power to nearly zero volts, the fault is typically metal like stray solder or a small piece of wire or other debris. Often that's repairable if you can find the material and remove it.

Sadly, when you get a voltage like 0.66 volts, that's typically a sign one or more semiconductors are shorting the power. Those problems are almost always inside the chips and rarely repairable. I'd probably not spend much time on that board showing 0.66 volts.
 
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