Hi,
TL;DR; how do you find the source of a 35 MHz interference/feedback signal, when it only appears when connecting two devices together? The devices works fine by themselves.
The longer story: I'm working on
v3 of my I/O card (entire schematic attached). I've got everything working; SD Card (both via SPI and MMC ifc) and digital & analog I/O via I2C. My last hurdle is the Wiegand interface for connecting standard entry/exit keypads. The specific part of the circuitry is as in the Wiegand.png image (page 3 in the attached pdf) - D0 & D1 run through a
TXB0102DCU to convert 5V to 3.3V for the MCU (ESP32 Wrover-B module). I've verified that the voltage translation works as expected; every negative edge on the 5V side on either D0 or D1 results in an interrupt in the MCU. I did simply by pulling D0/D1 high/low with a resistor.
The keypad I have is an
sTouch Reader R-W. From what I can tell, it gives a fair output, as seen in the image "DS1Z_QuickPrint1.png"
The problem is that as soon as I connect the keypad to the 5V side of the TXB0102DCU on my I/O card, I get the signal seen in image "DS1Z_QuickPrint2.png" and "DS1Z_QuickPrint3.png" on D0 and D1 (though only one line is connected in those images). How do I figure out where this interference/feedback signal comes from? Secondly, what I can do about it?
The measurement tools I have available are an unlocked Rigol DS1054 and Dave's 121GW multimeter.
PS: I'm already aware of that the NPN transistors are meant to be on the low side so no need to point that out. If you have constructive feedback on other changes I should do for v4, please let me know.