I've designed a board (HAT) that uses the ATmega328P microcontroller, and I think I might have made some mistakes in the board design. The design is most like the Arduino Pro mini, but with a 5A dual channel H-bridge, running at 3.3V 16MHz, and using the Raspberry Pi as the power source.

My Board: https://github.com/cescalemodels/BaseStationHAT/tree/master/Hardware

Arduino Pro Mini: https://www.arduino.cc/en/uploads/Main/Arduino-Pro-Mini-schematic.pdf

1) Do I have to have C3? I think it's for power stability and needs to be put next to the microcontroller, but I'm really not sure and want to eliminate it as a possibility as to which part made the board fail.

2) Do I need C2 if I'm not using the DTR Pin?

3) I assume that I don't need C13 and C10 because I'm getting power from the Pi, but can someone confirm?

If it helps, when I boot the raspberry pi, the serial port comes up, but I am totally incapable of communicating. Are there other components that I'm missing?

  • 2
    The 16MHz at 3.3v is the most likely candidate for failure...
    – Majenko
    Commented Apr 23, 2017 at 9:47
  • Capacitors are important, but I would atleast put the mounting holes/pads on your design and then just check if it works well withouth mounting the capacitors. If possible, monitor the voltage with a scope, to see if it dips when, for example, powering on the motors. But it's unlikely that they are the cause of the failure.
    – aaa
    Commented Apr 23, 2017 at 10:05

1 Answer 1


Every active chip should have adequate decoupling. That is the point of C3, to provide decoupling for the MCU. It is required.

While you don't need capacitors on the output of a non-existent regulator you do need a reservoir capacitor on the power input to your board. 10uF should be adequate.

No, you don't need the DTR capacitor if DTR is not connected to anything.

All that aside, there are two fundamental flaws I can see:

  • The raspberry pi can only provide something like 50mA on the 3.3v supply. (Or originally​ anyway, it may have improved with newer versions)
  • The ATMega chip cannot run reliability at 16MHz from just 3.3v. 10MHz is your top limit IIRC.
  • I didn't see any circuit in the links (just 5 files, which may or may not have been circuits). The PI B+ and later can supply 800mA from 3.3V, and ATmega328P will run at 12MHz on 3.3V (as used on Gertboard), although this is marginal according to the data sheet.
    – Milliways
    Commented Apr 23, 2017 at 10:09
  • @Milliways That's good re: the 3.3V. When the original board came out more 3.3V current was one of the many items on my list of recommendations to Eben. It seems that's the only one he took on board ...
    – Majenko
    Commented Apr 23, 2017 at 10:57

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