KenHahn

Darn it! I even proof read it before posting.

If you have a current limiting power supply, you could do some imperial testing to see at what current limit setting the issue is resolved to get a feel for what kind of baseline you may need to set. This guy put a lot of work into doing some...

Looking at the part spec, it looks like it is designed for USB 1.0, so only supports up to 12Mb/s which would be too slow for the Teensy USB 2.0 port that can run at up to 480Mb/s so that is no doubt affecting signal quality. Teensy doesn't...

Did you wire the D+ and D- to both pins on the USB-C so that the cable can be inserted in either orientation? If not, perhaps you just oriented the cable differently during your testing which broke the connections. D+/D- should look something...

Yes, adding this diode won't hurt anything and will help protect the USB host should you connect USB while you also have VIN power applied. The Teensy is good down to 3.6V, so the Vf drop won't matter. In fact it will take a bit of the load off...

As KurtE mentioned, 9V batteries have very little current capability of < 100mA. Your display needs about 280mA not to mention the Teensy which would like about 100mA all by itself. Find yourself an AC adapter of 7V to 12V to plug into that...

The cleanest approach would be to cut the traces so pins 20 & 21 are out of play, but it should work fine just adding the wires as long as you set pins 20 & 21 to inputs.

If it is failing right at power on, I would suspect that it might be something on the input power side. I have seen reports where people have a 5V DC-DC converter powering the Teensy and if they hot plug it in (rather than plugging it in and...

Is there any chance that the U1 3.3V regulator output is connected to the Teensy 3.3V pins as the pin labels seem to suggest? With a 5V VIN input, the Teensy 3.3V pins are outputs from the onboard regulator and should not be connected to a 3.3V...

You said the same circuit worked OK with Teensy 3.2 but not Teensy 4.0. I am guessing that something about the signal characteristics like the logic HIGH levels, signal edge rates or drive strength are a little different and one or more of those...

Those series diodes are only used for compatibility with 5V signal sources. They drop the signal level by about 0.72V and since you are driving it with 3.3V logic, the logic high level is only going to be up around 2.3 - 2.6V or so which is...