Custom MCU using a SMD ATMEGA328

I am building a custom low-power Arduino board using a SMD ATmega328 and as few components as possible.
I have drawn a schematic and want to get some opinions on whether this design looks good.


I have used the Gammon website as my base, however I am trying to use a SMD chip rather than a THT chip for space reasons, so I'd like someone to check it.

At this point I have a USB as power as I will be powering it using a 5v boost module which has USB output. I would be happier to incorporate the boost module into this circuit if I can, but I need to know if this will work first.

You will need to add the Boot loader to the controller chip before you can use the FTDI.

Nicks site shows how to burn the bootloader using the SPI (ICSP) pins.

Your circuit looks okay, make sure the crystal is close to the controller pins it connects to, do not run traces in between the crystal leads.

Suggest you add a 6 pin ICSP header to the board, at least the possibility of solder one if need be.

Decoupling caps need to be close to there pins too.


Suggest you just use an Arduino Pro Mini, they have all you need on the board.

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Yes. I'm going to follow the Gammon site for that! :wink:

Is there a way to upload the bootloader and then solder it to my PCB?? Or is that asking a bit too much?? :face_with_raised_eyebrow:

I need a few more custom capabilities than the pro mini - plus I want the charge circuit and boost circuit on the same PCB

Suggest you use ready built modules (daughter cards) on a Mother board that you design.

image

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Since you sound like this might become a hobby for you, go through this thread, it has ~ 1000 posts of ideas that might tweak some ideas.

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Here is an example of a Mother Board design with a Boost Converter, Pro Min and the capability of an DS3231 RTC.

From post

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Here is my schematic of a one-board 5v boost, battery, and MCU:

And here is the PCB.
Back:

Front:

Three things.

  • Unless you have above average soldering skills, a project using a TQFP form factor is not for you.

  • Making your own SMPS can be quite difficult, you need to use the recommended components (avoid substitutions) all recommended component placement is a necessity.

  • ready made boards/modules are inexpensive, less than what you can manage.

  • Suggest you use a boost/buck converter instead of a boost or buck converter.



Your vias should be enlarged.

Your cross-unders are too long and require return straps.

This is the foil pattern for your LM2621.

This isn't an issue as the company who is making the PCB offers a cheap guaranteed assembly service which I will be using.

Is this a switched mode power system?? It's just a battery powered device.

That is a fair point actually. This board is going to cost about $35, which is pretty pricey.

I don't want an adjustable output. I want a set 5v output, which is why I used a LM2621. Why do you suggest a boost/buck over this design?

I'm not entirely sure what you mean...

I did wonder about that.

β€œ Is this a switched mode power system?? It's just a battery powered device.”
A BOOST or BUCK converter is a switch mode power supply.


Traces in the GND plain (usually the bottom foil) are not recommended as they cause GND loop and noise issues.

This is especially true for long traces.


A cross-under is a trace on the bottom of a PCB, running through the GND copper pour.

Straps (to the top/component side) are placed across these cross-unders to correct for GND current flow problems that the cross-under causes.

image


That particular boost/buck converter does have a potentiometer on it which would be replaced by two resistors to get 5V.


A boost buck converter gives you power supply flexibility in powering your projects.

Example:

  • say you sometimes want to power the project by 3.7V, the boost part of the converter boosts the voltage to 5V.

  • say you want to power your board with 6V or 9V, the buck part of the converter reduces the voltage to 5V.

In fact as the battery dies you get 5V even when the battery voltage goes as low as 3V.


Anyway, these are just suggestions that you can take or leave.

That is a good idea to keep in mind for future projects, however I think this design should fulfil my requirements slightly better. According to the manufacturers datasheet, this chip will supply 5v even when the input voltage is very low (lowest working voltage is 1.2v. Whether it still outputs 5v at that voltage is the million dollar question).

I didn't realise that. That's very interesting.

As yet I don't have a GND plain - I forgot about that. Gonna go YouTube it...

What ever makes you happy, just passing on information :wink: .

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Thank you for taking the time to help me! I really do appreciate it!

@LarryD
Here is an updated PCB and schematic. I added the ground plane on the back, I'm sure it needs lots of work still though.



EDIT: Just replaced the PCB pics because I noticed a major issue.

Looks quite good !

The vias have a good diameter compared to the traces.

Power traces might be larger.

Notice the manufactures foil pattern for component layout.
You see copper pours for interconnections and not seen is the copper pour on the reverse side.

I honestly have no idea what I should be looking for in this - what is it?

Is this better?

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From your data sheet for the regulator, this is a proven foil pattern that the manufacture has tested and works for the components in their suggested schematic.

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