Feedback first PCB design coin cell thermometer

[joelucid]

Guys,

after all the listen mode / coin cell discussions I've finally cut my teeth into Eagle for the first time today and put a design together. My goal is to create a PCB that is small enough to use in tiny thermometers, but that can also be used more generally in battery motes.

I've tried to be as minimalist as possible - so no reset cap or pull-up and I'll drive the I2C bus from the internal pull-ups. I don't have any bulk caps since our discussion has shown that they are likely not required.

To be able to use this for non thermometer projects I've added two headers with the most important pins I might use. Unfortunately I had to add them both on one side to avoid further coin cell overlap on the right edge. This means its not breadboard compatible and likely a little less mechanically stable than it could be - but I think I can live with that.

I'm planning to preflash the 328p with my bootloader so I don't have an ISP header. I do have all required pins for ISP on my headers for emergencies though.

I've positioned the components so that the decoupling caps were close to most chips and oriented the avr naturally for the headers. Then let the auto-router do its thing. It doesn't look pretty - but it seems to be complete.

These are my first baby-steps in HW land - so I'd be very thankful for any feedback.

Thanks Joe

[TomWS]

Oh Boy!  THIS is gonna be fun!!!   

A couple of quick comments but I'll take a deeper look into the board layout and maybe make some adjustments and send them your way.
1. I highly recommend a filter cap on AREF.  0.01 and 603 size should be fine.
2. I've never had success on an AVR design without a pullup resistor on Reset.
3. I'm not sure SS on JP2 is of much value and you will ALWAYS need a ground to make any of the header signals useful.  For ISP you'll also need Reset and VCC (maybe).
4. I'll add a ground pour on both sides as this will help ease some of the routing issues and make for a more solid ground distribution.
5. I think there's plenty of room to scooch the battery so it doesn't hang over the edge of the board.  Is there a reason you placed it exactly so?

That's enough for now...  More later...

Tom

[joelucid]

Hey Tom - thanks for your quick response!

A few quick comments:

Rg 3: SS shouldn't be there you're right. I was under the impression it was required for ISP - but I guess that's what reset is for in that case.

Rg 5: The two leftmost squared contacts are for the + battery connectors. If I move the battery left the PCB needs to get wider which is worse than having it overlap over the edge. I don't like the overlap but I like the square form factor. The GND battery connector is positioned between J2 and J3 to enable the square design despite of the header contacts. If you don't use a coin cell it becomes the GND header contact. I could use the former SS pin as second GND for use with coin cells. Realistically though you're not likely to use headers with coin cells for size reasons - rather use the contacts directly for some extensions.

[TomWS]

I sent an updated BRD file to you (with the battery moved, and it still fits 

Adding another hole for a header type ground connection shouldn't be a problem, but you can decide what's important to you.  I think the biggest thing is to use ground pours and a few well placed ground vias to get good ground distribution AND better routing of your other signals.

Tom

[joelucid]

Thanks Tom,

I looked at the changes you've made and updated the schematic to add a pull-up on reset and the aref cap. Then I rerouted everything keeping in mind your feedback. The result is attached. Could you take a look and check whether it looks workable?

One thing I find a bit concerning is the VCC routing if no coin cell is on board. That's a pretty long journey across both sides of the board. Might not be an issue because of the decoupling caps nearby.

Thanks Joe

[TomWS]

Sorry, just saw this.  I'll look at it later tonight or tomorrow morning.  I wouldn't think VCC trace length is an issue on 25mm board, but I'll check.

Tom