ATxmega developement board

[kalan01r]

Hello,
   I decide to create xmega dev board. I do not have much experience, so i would love to hear comment on my design.
   Goal was to create board around ATxmega256A3U, with dc-dc power supply. I decide to add some simple protection for power supply. Internal clock allow uc to run at 32MHz, so i think that external crystal should be 32.768khz. For programming PDI connector, and usb with DFU button. Port F is connected via buffer to led. That is basically all. I have no idea what more i can add.

Project is here : https://easyeda.com/kalan01r/ATxmega_protoboard-395da7c0d7f64d3a8896c233dbb05919

[necessaryevil]

I think you can make it cleaner by less swapping between layers. For example, in port A, you start some layers in red, then you swap to blue and then you swap back. Why don't you go to layer blue immediately after the MCU? You can stay there.

[kalan01r]

I think You are right. When i took second look at the board there really is no need to stick to red layer in that area. Cutting ground is no problem there, because there is no need for ground there.
 In project there is second pcb with changes i made.

[necessaryevil]

Yup, that is what I meant.

If you want to use a reflow oven it's not a good idea to have SMD components on both sides of the board. The components facing downward will fall off.

[David Chamberlain]

Yeah I would tend to agree with @necessaryevil, you may better to drop all of your signal lines as they exit the MCU to the bottom layer that would give you plenty of routing space with the decoupling caps on the top layer and you can move the XO closer to the chip at the same time It may make your PCB size smaller even.

Double-sided reflow can be done, and I've done it before - the BAD way - by re-flowing one side letting it cool and press a single sheet of tinfoil against the loaded parts to stop them falling off then do the second side. Or use a glue to stick them down and do both sides at once (preferred method)

[ajb]

Nah, you don't need glue, or really any special procedures at all for a typical double-sided load.  The solder itself will provide sufficient holding power for chip components, ICs, etc--really the vast majority of typical SMT parts--on the bottom while the second side is reflowed.  The only problem parts as far as being at risk of falling of the bottom of the board are going to be especially heavy ones with relatively small pads, such as big inductors (heavy) or large electrolytic caps (small pads). 

There are formulas that describe the relationship between allowable weight and solder joint area that will provide some guidance here.  Here's one relevant paper found with a quick google (PDF): http://www.circuitinsight.com/pdf/weight_limit_qfn_smta.pdf

[kalan01r]

I'm going to hand solder it, so in that case it's not a big deal. But I'll keep in mind what You said for future projects

[thm_w]

Looks good.
You seem to be missing designators for the connectors (like J1, P8, is not shown on the silkscreen).
Also normally for connectors (P4) I would include gnd and possibly vcc in each header, but it depends on what you want to use it for. I see you have P8 for plenty of gnd/vcc connections.
Polarized headers (like you have for PDI) are nice but take up a lot more space.

[kalan01r]

On the silkscreen i would like to have only information label (port, vcc, gnd,etc). My intention was to remove designators like J1 and P8, to have clean view on what user can use. I was thinking about adding power connection to ports, but at the end i decide to create one separete power connector. I will see how it will work in practice, and mayby ill change it later.

[davep238]

I am puzzled by S1, C14, and C15. Why does the switch connect grounded caps to port pins? If the pins are inputs, it looks like they will float regardless of the switch position.

[kalan01r]

PA0 and PB0 in ATxmega256A3U can be used as AREF. In case ill put external reference voltage there it would be nice to have those caps. In normal use they might be problem. That is the reason why i add switch.