My circuit uses LAN8742A ethernet phy. The system has a metal case and gets 220V AC to power supply board. After converting 5V DC it goes to my board that has Ethernet and mcu. I screwed case and power supply pcbs earth point to mains earth. ethernet phy nrst pin connected to mcu nrst pin and 1 capacitor and 1 extra pull up resistor for guarantee. When i hit the ESD to metal case system fails. I found the problem point that is nrst connection. Not EMI but RFI affects it. When i cut the connection of ethernet nrst pin between mcu nrst pin. (ethernet nrst pin can float cause it has internal pull up) system works fine. My tries to solve:

  • changed capacitor value(inc&decr) ✘
  • changed resistor value(inc&decr) ✘
  • removed capacitor ✘
  • removed resistor ✘
  • changed capacitor to TVS(uni&bi) ✘
  • added TVS to power input ✘
  • increased power input capacitor ✘
  • all combination of theese ✘

Apparently I will remove this connection on next revision of pcb. I dont have to connect ethernet nrst pin but i wonder what is the reason of this situation? My circuit scheamatics are: MCU ethernet power SWD


1 Answer 1


C14, at 0.1uf 1206, is going to resonate around 20MHz. For higher frequencies (such as those radiated by ESD spikes) use a smaller one, say 100pF, closer to the IC, and put a ferrite between them (to provide additional filtering in addition to decoupling the caps enough to avoid tank resonances).


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