Hi,
I'm working on my first larger circuit and so far things are working out above expectation, but I've got one issue I can't find the cause of; whenever the USB cable is connected to the MCU breakout board (
Sparkfun ESP32 Thing), I get severe signal distortion in two of my input lines, see the attached images.
The input signal is a 5V active low signal (Wiegand signal from a keypad, powered from the same 12V as the rest of the circuit). The first image shows what it looks like with the USB connected, the second one with only the device's own power supply running. This is measured on pin 1 & 8, left side of U401 (
TXB0102DCU) in the middle of the schematic.
As can be seen in the third image I put a jumper (JP401, center left of the MCU on the right side in the image) for the 3.3V power of the MCU to be able to cut it off from the internal power while the USB is connected, but the distortion is present even with that off.
Could someone please give me some pointers what I've done wrong?
The entire schematic is available
here, in KiCad format, and also attached as a PDF to this post.
Update: Connecting a Laptop running only on batteries, i.e. isolated from mains results in the same distortion.