Postage Stamp Micro-controller

In a previous post I described a little one inch square micro-controller based on the ATMega328 which I decided to build in order to learn KiCAD.  I have the boards back from OshPark.

To my way of thinking, it is a pretty good bargain to be able to obtain this lot of boards for the grand total of $10 plus shipping.  In any event, I have built up one of the units as seen below.

I have a couple of errors I will need to fix.  The first is that the row of header pins on the left and right side are not quite on 0.1 inch centers.  I plugged the headers into a prototype board while soldering and they both bend towards the centre of the board, so I need to space them a little closer together.  Oops...  The top and bottom headers are mounted on the top side of the board to allow plugging the board into a prototype board and still having all pins available.  I would expect to typically hard-wire the board into whatever circuit it will be employed in and then mount it down with double sided tape.  This one however is for bread-boarding.

The second error is that I used the wrong footprint for the thru-hole crystals that I have on hand and so it cannot sit completely down on the board.  Oh sigh...  Easy enough to fix however and I will make those updates before I publish the board at OshPark.com.

The ATMega328 chips are raw ones with no boot loader, so I will have to program them with the ISP header.  This can be done with another Arduino or any of the ISP programmers such as the USBTinyISP.  No boot loader is required on this device as there is no USB or other programming interface other than the ISP connector so therefore the entire flash is available for your code.

KiCAD

Since being laid off at Dynon Avionics, I have been looking around at schematic capture and PCB CAD packages.  Chris at Contextual Electronics has encouraged me to try out KiCAD.  I decided to give it a go and design a small 1 square inch microcontroller board with a full-on ATMega 328 processor and the minimum circuitry necessary to build an (almost) UNO clone.  The "almost" part is the fact that there will be no USB port on the board and it will need be programmed using the ISP connector.

The circuit diagramme I decided on looks like this from KiCAD:

Nothing here that doesn't need to be here for a minimal controller.  I have the entire I/O pin set that can be found on an UNO, plus since I use the TQFP package, I have two additional analogue input pins.

The board layout is pretty dense at one inch square.  Here is the board layout showing all of the layers from KiCAD.  I am not terribly impressed with KiCAD's copper pour capabilities, at least as far as I have explored them, but what do you want for free, eh?

Producing the Gerber files was trivial and I zipped up the lot of them and shipped them off to OSHPark to be manufactured.  Here is the screen shot of their rendering of the gerber files for the front and back of the board.

So, now we wait for the little purple envelope to arrive.  Meanwhile I will collect up the set of parts to build a few of these up.  This was an experiment to enable me to learn KiCAD and at the same time to produce a useful gadget for other projects.  Once I have verified the functionality is correct, I will publish the OSHPark boards so others can order them directly if desired.  At $5 for 3 of them, they are pretty cheap.

Overall, I found KiCAD relatively easy to learn, once I got a couple of key concepts down.  The main one was that hot keys apply to whatever your mouse is hovering over.  I found myself wanting to click to select something before hitting a hot key and this lead to all sorts of grief.  Moving objects once they are placed also needs a lot of work.  Rubber-banding of traces pretty much sucks.  Once I figured out how to drag an object, the best that KiCAD could do is straight line connect all the existing traces.  This ends up being pretty much worthless for anything other than adjustments of a few centimetres and you end up deleting all the traces and redrawing them anyway.

Selection of wires in schematics needs an easy way to select only a line segment or the entire line without having to draw box around it.  When you move the line, it needs to rubber-band the end points.  Straightening out a series of segments needs to collapse them into one segment and it needs to be able to easily break a line segment in two without disconnecting them.  All of these comments also apply to traces in the PCB layout editor.

I did like that KiCAD included the ability to view gerber files, a comprehensive footprint editor as well as building the notion of hierarchical schematics into the product as a key feature.  I cannot however get the bill of materials functionality to work, so I must be doing something wrong.  Always more to learn. 

For a free, open-source product, it is quite compehensive and there are a lot of part footprints available out-of-box and from 3rd parties.  Overall, I would continue to use it for hobby projects.  I would need to try and build at least a four layer board to have an opinion about suitability for commercial projects.

VNA fixture PCB available from OshPark.com

Due to a number of requests to share my VNA fixture design, I have made them available at OshPark.com using the following link.  Boards can be ordered directly at that link in multiples of three very inexpensively (about $11).  Drop me a note at ko7m at arrl dot net should you encounter any difficulty in obtaining them.

The tactile switches used on the board are available at mouser.com at this link.  As always, I would be happy to help you succeed in implementing this board should you so desire.