Hi everyone,

I’m working on a battery-powered data logging project using an Arduino Pro Mini. The current prototype uses the Pro Mini with a DS3231 RTC module and a microSD card (SPI) to store timestamped sensor data. Since the breadboard prototype is working, I’m trying to design a custom PCB using the ATmega328P directly instead of the Pro Mini module so the board can be smaller and optimized for low power.

While studying the Pro Mini schematic and starting the PCB layout, I ran into a few questions:

1. Clock configuration – The Pro Mini comes in both 8 MHz and 16 MHz versions. For a battery-powered design, is it better to run the ATmega328P at 8 MHz with the internal oscillator, or should I still use an external crystal for better stability?
2. Programming interface – Since the Pro Mini is usually programmed through an FTDI adapter, do I only need to expose the TX, RX, VCC, GND, DTR, and RESET pins on a header to keep the same programming workflow?
3. Decoupling capacitors – The reference designs show multiple 0.1 µF capacitors near the ATmega328P power pins. How critical is the placement distance of these capacitors from the MCU?
4. PCB mounting holes – I added a couple of mounting holes to fix the board in a small enclosure. Should these holes typically be connected to ground or kept electrically isolated?
5. Minimum drill size for vias – My PCB tool currently uses 0.25–0.3 mm via drills. Are these sizes generally safe for standard PCB manufacturers, or should I increase them to avoid fabrication problems?

Since this is my first time designing a custom Arduino-compatible board, I’d appreciate any advice about the ATmega328P setup as well as small PCB layout considerations that beginners often overlook.

Thanks!

 

For a battery-powered ATmega328P board ..... running at 8 MHz with the internal oscillator is usually best for low power, though for precision external oscillator is recommended .... read the cons of internal oscilltor: https://forum.arduino.cc/t/cons-of-using-the-internal-8mhz-oscillator/690931

I think exposing TX, RX, VCC, GND, DTR, and RESET on a header is sufficient for an FTDI workflow. Place a capacitor of 0.1uf to the MCU power pins.

Mounting holes can be non-plated for mechanical support or plated and tied to GND for shielding. Vias of 0.25–0.3 mm drill with ~0.6 mm pads are safe for most PCB manufacturers. I normally order on pcbway with these specs andnever had issues

 

For a battery-powered ATmega328P board ..... running at 8 MHz with the internal oscillator is usually best for low power, though for precision external oscillator is recommended .... read the cons of internal oscilltor: https://forum.arduino.cc/t/cons-of-using-the-internal-8mhz-oscillator/690931

I think exposing TX, RX, VCC, GND, DTR, and RESET on a header is sufficient for an FTDI workflow. Place a capacitor of 0.1uf to the MCU power pins.

Mounting holes can be non-plated for mechanical support or plated and tied to GND for shielding. Vias of 0.25–0.3 mm drill with ~0.6 mm pads are safe for most PCB manufacturers. I normally order on pcbway with these specs andnever had issues

wow thanks for the prompt reply, Regarding the oscillator, I’m not too concerned about precision for my data logging, more focused on battery life as its my main priority, so it sounds like the internal 8 MHz oscillator makes sense. thanks for this discussion link, will check it out.

About pcb ordering, you mentioned ordering boards with 0.25–0.3 mm vias and 0.6 mm pads, did you ever run into any issues with small components or panelization? I’m planning to order my first batch and want to make sure these specs are safe.

 

Yes, in your case ..... the internal 8 MHz oscillator is perfect ..... I’ve used it on several battery-powered boards without any issues .... precision issues come when there's too much processing load ... neve happened to me though

On PCBWay, I’ve ordered multiple small boards with 0.25–0.3 mm vias and 0.6 mm pads, including fine-pitch SMD components, and never had any fabrication problems. Just make sure to double-check your DRC and keep adequate clearance for traces and pads, and you should be fine for your first batch.

Check out these guides on DRC rules:
https://www.aivon.com/blog/pcb-design/troubleshooting-common-pcb-design-errors-a-practical-guide/
https://www.pcbway.com/pcb_prototype/PCB_Design_Rule_Check.html

 

Yes, in your case ..... the internal 8 MHz oscillator is perfect ..... I’ve used it on several battery-powered boards without any issues .... precision issues come when there's too much processing load ... neve happened to me though

On PCBWay, I’ve ordered multiple small boards with 0.25–0.3 mm vias and 0.6 mm pads, including fine-pitch SMD components, and never had any fabrication problems. Just make sure to double-check your DRC and keep adequate clearance for traces and pads, and you should be fine for your first batch.

Check out these guides on DRC rules:
https://www.aivon.com/blog/pcb-design/troubleshooting-common-pcb-design-errors-a-practical-guide/
https://www.pcbway.com/pcb_prototype/PCB_Design_Rule_Check.html

Sounds good, I’ll double-check the DRC and clearances. Btw do you usually panelize small boards like this, or just order them as single units? I have around 30-50 pcs