I killed my Teensy 4.1

Projects General Guidance
1

Hey everyone, I'll try to keep it as short as possible. I hope this topic is OK even tho it's not about any Arduino board.

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 - multiple times previously with no issues. However, once while unplugging from the USB, i didn't power off the 74hct244 and 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.

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?

2

Active signals into unpowered parts are generally a recipe for disaster. To misquote Ian Malcolm, "Current will find a way."

3

Alright, so always power off the external 5V first THEN the Teensy? I haven't even thought about something like that in the first place to be honest, didn't realize the 5V from the 74hct244 could affect the unpowered output from the Teensy.

4

If you do that, you still have powered parts connected to unpowered parts, do you not?

5

Only 3.3V logic with minimal current will flow into the unpowered 74hct244. The ESP32C3 is also powered from the same 5V but only sends data to Teensy, doesn't receive any (and also is the same 3.3V logic as Teensy, so I'm not sure if that's a problem).

This is my diagram - those 2 chips are step-downs (for the 5V). The PC connection is not in the diagram, but you get the point. Also, the chip is not called 74hct244, I just needed something that looks the same:

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Is it incorrect? Is there any other way to do it so i can still use the USB connection while using external power source for other electronics?

6

I think the Teensy not lighting up and getting hot answered the "is it incorrect" question already.

I believe I've answered the question of why it failed twice now. You cannot have powered and unpowered parts connected together. Dance around it all you like, the answer won't change no matter how much you want it to.

Power the Teensy from the same power as everything else. The little reference card that should have come with your Teensy (and is available on the web if not) shows exactly what to do.

7

Ok, sorry, just wasn't sure if your previous reply wasn't a question. I'll try to look for different solution then.

8

It doesn't matter is the signal a 5v or 3v3, you cannot apply it to unpowered board

9

Yes.
Use buffers with the Ioff feature on all connections between the teensy and external devices. Power the buffers from the Teensy

10

Correct. Also, logic level shifters are required to safely interface 5V logic with 3.3V logic. It is much better to stick with 3.3V logic for everything, as 5V logic will soon be a thing of the past.

Further analysis of the problem requires that you post a complete and CORRECT schematic diagram of the circuit, not something that "looks the same". Hand drawn is fine, and much preferred over Fritzing diagrams.