I killed my Teensy 4.1 - what did I do wrong?

[vorisek_]

Hey everyone, I'll try to keep it as short as possible.

I had Teensy 4.1 connected via USB to PC so I can feed data from my video software to it. Teensy was then sending data to 74hct244 which was powered via external 5V. I had some issues with the program I was using to send data to Teensy so I unplugged Teensy from PC and replugged it WITHOUT unplugging the external 5V feeding the 74hct244 - few times previously with no issues. However, once doing so, suddenly the Teensy doesn't light up and, even when not processing any data, gets kinda hot.

I'm certain I've fried it, BUT I would like to ask for the possible cause which killed it? I also had ESP32C3 connected to the Teensy only via serial line (TX-RX) to send control data from potentiometers and it stopped working as well.

My guess is the 74hct244 either died first and then 5V from the external power traveled back to Teensy and also to the ESP via the serial line, or it's just a mistake overall to unplug the Teensy while still powering the 74hct244 with external 5V? How does the 74hct244 behave when there is no input (Teensy powered off)?

I could be totally wrong in both of my assumptions, I just don't want to make the same mistake again.

What do you guys think?

PS: My diagram for reference - missing the PC connection and also the 74hct244 has different name. Also there are 2 step-downs from 12V to 5V.

 

[PaulStoffregen]

Any chance the pink wire accidentally touched the red 12 volt wire? Looks like they connect to adjacent pins on a terminal block.

 

[vorisek_]

Thanks for the reply!

I don't think so, I'm using 3.81mm pluggable terminal blocks for this - no signs of contact whatsoever.

However as I've been researching, I found out the likely cause was solely feeding the 3.3V data into the UNPOWERED 74hct244, which can damage it, so I guess it just couldn't handle it anymore and failed, but I'm still not really sure. What I get from all of this is to power everything from the same power source no matter what so no unpowered hardware gets fed data voltage into it - which is something I've honestly never though of.

So should I maybe use the same step-down to 5V to power the Teensy externally and cut the VIN and VUSB pins on the back of the board, because I still need the USB for data? What do you think?

 

[MarkT]

CMOS latchup is common in this sort of situation (phantom powering).

 

[Frankthetech]

Yep, back feeding is a problem. I had a project where the device I connected to had it's own power system. Everything went fine till someone thought my project should be turned off at night but left the connected device powered. This resulted in 5 volt feeding into the teensy3.2 circuit through it's port pins.
The teensy failed (I don't know how long it took) So now I put a 1k res on any line that connects to another circuit, 5/1000=0.005 that will only allow 5 ma to flow into a port pin should the situation happen, I know that will limit the port pin drive, but it seems to work fine for comm signals.