Design Guideline Help

Hello all!

I am pretty new to PCB design. I am using Kicad and I have taken another user’s board the he posted here and redesigned it to open up some space for more components. Learning Kicad was fun.

I have attached a picture of the top of the board and the back of the board

I have crammed all of the components on the opposite side of the chip ATSAM3X8E, and I was wondering if by doing this I will incur the wrath of a signal interference from the chip on the other side. For example, there is a crystal and a resonator that are on the reverse side of the CPU, will the normal operation of these be affected by the chip?

In general, are there design guidelines I should be following for placement of components? This is something I have had no experience with.

Thanks!

Probably no effect. Those frequencies are relatively low so they shouldn’t radiate too much interference. If you were really worried, go to a 4-layer board with a full ground plane on one layer.

Is there a reason why you have a ground plane on one part of the board? Sprinkle a few vias in the tracks that are constraining that copper pour and let it spread out to cover more of your board. Let it get under the crystal if you can.

I don’t see any silkscreen in those renderings. Make sure you label the reset and erase buttons in a way that’s visible in the manufactured PCB. In the worst case, use copper tracks to write the words.

I’ve already used up most of my PCB layout advice in this thread.

Thanks for the reply.

The ground plane shown in the pictures is actually a VSSA plane, which only covers the analog pins and traces. Maybe this is something that helps isolate the analog lines? Would that make any sense?

Also another question: in Eeschema (the electrical diagram part if KiCad) all of the VDDcores / Voltage lines are connected together, and then there are an equal number of capacitors (one for each lines) off to the side that go to ground. Does each line need a capacitor RIGHT NEAR each VDDcore pin? Or can they just be somewhere on the board as long as they are all connected together? The latter doesn’t really make sense to me, but that’s the way I’ve done it on the board.

All the capacitors are lined up and connected to each other, with an indirect connection to each VDDcore pin. Hopefully my question makes sense.

You need the caps adjacent to the pins they are decoupling.
Ideally no traces, especially power traces, under the crystals. Slow, rarely changing signals, like Reset, are ok. See the crystal section of this app note, would apply to the SAM chip also.

AtmelAVR042 AVR Design Considerations.pdf (236 KB)

Crossroads. Thank you very much for the guidance. I figured I would have to put the capacitors near the pins. Time for some rework.

I have both a crystal and a resonator on the board (I think). Do I need to keep traces out from under both of them? Or just the crystal?

I'll take a look at that pdf. Thanks again!

Keep this in your reference section, note page 6-7.

http://www.analog.com/static/imported-files/tutorials/MT-101.pdf

An issue that concerns me is that you have a through-hole crystal on top of the processor chip. The crystal's pins will be protruding through the other side of the board, preventing the processor from sitting flat. Use a surface mount crystal, or move it out of the way.

Keep in mind that through-hole components always interfere with both sides of the boar: when placing them, make sure there is clearance on the other side.

Thanks everyone - I will make these changes!

Hello all,

I have made a lot of changes - used a SMT crystal instead of the thru-hole, moved all my decoupling caps right next to the voltage pins they are decoupling, and I’ve added a ground plane and power plane, making it a 4 layer board.

One question I have is now that I’ve got a ground plane, I’d like to split the ground plan into analog and digital. I’ve attached a picture of the separation between the two. Does the gap between the two look sufficient?

Thanks!

internallayer.JPG