I'll disclaim this by saying I'm a very green when it comes to electronics, know enough to be dangerous, that's about it.

With that said, let me explain a little bit about my project.

I'm working with automotive electronics and CAN bus, I'm using an ECU Master PMU-16 (https://www.ecumaster.com/products/pmu/). My basic understanding is that it takes an input voltage, and forwards that voltage to other devices employing some current limiting along the way using a mosfet(s). The outputs are switchable, through a couple of methods, and one is CAN messages which is where the Teensy/my project joins the party.

Here's a basic diagram of my circuit, I hope it makes sense (see disclaimer).

VCC in this instant is my bench top power supply, currently outputting 13.8v.
GND is the negative terminal on the same PSU
There are two nodes on the BUS both terminated with 120ohm resistors.

In the above circuit if the PMU is powered before the Teensy, then nine times out of ten the Teensy will not boot. In a non boot state, ~5v can be measured across the VIN and GND pins.

If the Teensy is powered first, and then the PMU all is well. IF I remove the SN65 from the circuit, all is well. But I need the SN65 and CAN for my project to work, it does seem to point to the SN65 being the issue.

If I fit a diode between Teensy 3.3 out and 3.3 in of the SN65 my problems go away, meaning I can power the Teensy after the PMU and everything works as expected.

You might say, well you've fixed it, so what's the problem?

Well it was a bit of a guess, after measuring 3.3v at the SN65 without 3.3 from Teensy connected up. So a better understanding of why that fixed it would be great, please.

I also might have ordered some circuit boards based on the original design without the diode, so if you can think of something that means I don't need it, that would also save me some time/money/difficult soldering job, haha!

Hope the above makes sense, and thanks in advance.


PS. I did see this https://forum.pjrc.com/threads/58019...-circumstances which sounds quite similar to my problem, they solved it in a different/better manner? I also don't know for certain what transceiver is in my PMU.