I am working on a PCB to enable an ATmega328 to connect to an MQTT network through an ESP12F. I am using a voltage divider in the TX line from the ATmega to reduce it to 3.3v (using nets SR+ and SR- for the communication,) but I am having difficulti to maintain the communication. Most of the time a power reset solves the problem.

What am I doing wrong?

Attached are the schematics:


  1. How can I solve this issue?
  2. what considerations i need to make while routing these communication lines.
  • \$\begingroup\$ Most likely AVR powered from 5V can't reliably receive 3.3V signals. Or a code issue. Why are you not running the AVR at 3.3V too? There is also a weird capacitor between CH340 V3 pin and 3V3 supply, what's the purpose? \$\endgroup\$
    – Justme
    Commented Jan 20, 2022 at 14:51

1 Answer 1


Your power connections are incorrect. The V3 cap should be in parallel if 5V is used. You are using 4.7V for some reason when the chip works down to 3V and you have a 3.3V regulator in series with a cap to V3.

You should have C8 going from 3.3V to ground and connect Vcc to V3 to 3.3V.

*Reset is a C-coupled output is a mystery or a mistake.

  • \$\begingroup\$ 1.I am not sure how to connect the V3 pin. I saw many different versions of the V3 connection (GND, 3v3, and 5V). so I made a 4 pin header with all possible connections for testing, if needed I can jump the capacitor \$\endgroup\$ Commented Jan 21, 2022 at 6:47
  • \$\begingroup\$ i got these schematic references from Arduino schematics so the RESET connection I got it from them, not sure about this \$\endgroup\$ Commented Jan 21, 2022 at 6:48
  • \$\begingroup\$ Recheck and list your sources in your question. Obvious errors. \$\endgroup\$ Commented Jan 21, 2022 at 7:07
  • \$\begingroup\$ What changes i need to make if I want run completely in 3.3V \$\endgroup\$ Commented Jan 21, 2022 at 7:30
  • \$\begingroup\$ Do as I said.... \$\endgroup\$ Commented Jan 21, 2022 at 7:31

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