Arduino pro mini - simplify schematics

Hi there, for a project I want to design a pcb which incldues an arduino pro mini (3.3V/8Mhz) and a LoRa Ra-02.
I’m very inexpert with schematics (but I’m trying to learn) and new to KiCad. What I’m trying to do is basically to simplify the arduino pro mini as much as possible but I’m not sure exactly of what I can/can’t remove.

The schematics I have are taken from here.

With my limited understand I’ve removed a few things (pretty much everything) except for:

  • the crystal
  • the C3 component between GND and VCC (no idea what is it for)

This is assuming that I will power directly the ATMega328 with a battery (ATMega Vcc accepts 1.8V - 5.5V from datasheet) likely 3.7V (for the LoRA module to work correctly).

The resulting schematic is below.

My questions:

  • can I remove the C3 component? What is it?
  • Is it safe to use the battery directly to the ATMega or is it better to use something to protect the chip? In which case, what would I use?
  • I see pins for Vcc (18,4,6) and GND (5,3,21) are all connected together. Is there a reason for it or can I just only connect 1 of each to Vcc and GND? for example disconnect pins 5,3 and 6,4, leaving only pin 18 to Vcc and 21 to GND.

Any comment is much appreciated!

FYI at the moment I have the LoRa working well with the arduino pro mini board so I’m basically trying to design a pcb to put everything together without cables (and smaller size).

Thank you!

This site is for learning and using Kicad. The Arduino forum would be the correct place to ask questions about the nuts and bolts of the functions of the hardware. You might get answers here but as volunteers we try and keep focused to keep the work load down on those maintaining the site. :wink:


forgive me, in my mind it was more schematics than arduino like (at the beginning) and then it turned out to be much more arduino focused than kicad related but I didn’t realize. Thank you

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Are you planning to incorporate an Arduino ProMini as a daughterboard on your project or simply design a bare bones Atmega 328 based board with some peripherals? I presume from your comments the later but you start off talking about an Arduino.

The capacitor C3 is required for PSU decoupling and needs to be close to the board.

I would definitely run this with a voltage regulator - sure, you can set brownout fuses etc but if it starts getting flaky, can you be sure its not the power supply? Power supply stability is fairly fundamental to reliable MCU function.

There is equipotential bonding within the chip - but the connection is not designed to deal with higher power requirements. You may get away with it but I wouldn’t even try. I would design the board with as continuous a ground plane as possible which will achieve this simply.

How are you programming this? You will need to provide a programming header and probably a reset mechanism.

Have you considered putting a pre-built ProMini directly on your PCB as a complete device - i.e. a daughterboard as mentioned earlier? You say you are lacking space so this may not work but this would solve some of your design problems - especially if you have limited design experience. This would add USB programming and the reset mechanism for free. There are quite a lot of design considerations on a project like this and having the MCU side sorted as a monolithic block might be a good starting point. Building in the castellated LoRa module also has some design and positioning considerations.

No worries. My post is more about the fact you’re more likely to get the help you need at the Arduino forum. It’s pretty active. I don’t know about the mini but there are designs around for doing the bare basic Arduino on a breadboard.

If space matters there is always VQFN and smaller :sunglasses:

Hello, the idea was to desingn a bare bones ATMega 328.

What do you mean by “close to the board”?

ok does it still count if I only power the system with a battery? I mean, obviously I assume a regularly charged battery (e.g. 3.7V Lipo). I was reading the voltage regulator is one of the component that could be removed to save power (but to be honest I’m not sure if the power saved by removing it would be particularly relevant)

Forgive me, I don’t understand a word about this

I have FTDI, I’ve removde the pins (from above) but I’ll re-draw them later.
Reset button I don’t know if I’d use it, if I can reset the board via software that would probably enough…but I guess I’d need DTR with a capacitor…probably it’s more useful to have RST button than DTR?

Yes and that’s my middle step in the design but I’m trying to design the final pcb (no daughterboard) as well.

pardon I have no idea what this is. Could you expand?

Thank you all!

Hello, the idea was to desingn a bare bones ATMega 328.

OK, understood.

What do you mean by “close to the board”?

I mean the capacitor should be as close as practically possible to the Vcc supply pin on the Atmega chip and the other end connected by the shortest path to a ground - ideally via a solid ground plane. (When I said board, I meant chip mea culpa)

only power the system with a battery

You could try it on a breadboard. You need to be certain that you won’t accidentally connect a higher potential (and burn out your chip) and not care if it drops out in an unpredictable fashion. What else are you expecting to power from the battery. Voltage regulators are pretty small, efficient and cheap. How small does this board need to be?

equipotential bonding within the chip

The pins marked GND, for instance, are all connected together within the chip. However, the connections are not designed to pass the total current limit for the chip. If you load it to its rated maximum and are only connected via one GND pin, you may burn the chip out.

VQFN package

Just for kicks :sunglasses:

Now that’s a bare bone 328PB without silly pin mapping. Integrated +5V from single AA cell.


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