"Raven" Microcontroller host board design: ver. 2

[tetsujin]

After discovering some serious flaws in my first version of the "Raven" microcontroller host board (among them being that it only had enough free I/O lines to control three LEDs) I decided to try a new design...

(If you want to see the old design, and the space limitations that led me to start this little project in the first place, it's here...)

Instead of an 8-pin part, this version uses a 14-pin part that's the same size (thus harder to solder, too...) The current WIP version provides five I/O lines usable for driving LEDs. There's still some problems I have to solve, however.

The main limitation at this point is that these outputs still can only drive or sink 25mA each - and only at about 5V. This means I can't run two white LEDs in series, and I can't put more than one of them in parallel on a single pin (not at full intensity, anyway...) So I think I need to come up with some kind of LED driver add-on that I can sandwich onto this board.

The basic idea of this board, in a nutshell:

• Very small - about 1cm x 1cm
• Designed to simplify and incorporate as much of the external wiring as possible (for instance, there's on-board provision for series resistors for simple LED applications)
• The software on it could vary depending on the model in which I use it - but generally I expect it to be driving LEDs at different intensity via PWM
• Next to the power connections there are clock and data lines: these are to be used to allow external circuits to control the board's behavior remotely.

When the design is complete I plan to use it in a lot of my projects - as the default circuit of choice any time I need a microcontroller.

[tetsujin]

Pity I screwed up...

This is my first time having a circuit board fabricated... Wouldn't you know I'd neglect little details like correctness... I ordered the boards around the beginning of the month, discovered my mistakes over the weekend, and the boards arrived today...

Still, the boards are usable - maybe useful, even. But I think, having seen how this batch turned out, it's time for another round of redesign...

[tetsujin]

More new "Raven" design versions...

I'm getting ready to order another run of boards: I've made a few attempts to rework the board design to include more features while making the board smaller: I had done some 9mm-wide versions of the design but it's been very tough to make these work, so at the moment I'm focusing on 10mm wide versions. I think the latest version I've done, version 6, is very promising: it's a 10mm wide board with all I/O pins accessible, with series resistors for all I/O pins apart from those in the ICSP header, and with pull-up resistors for the chip's built-in synchronous serial port support, and a pull-up and capacitor for the chip's reset line. The v6 design starts at 10mm x 12, and can be cut down to 10mm x 9mm (or maybe 10mm x 8mm) by sacrificing extra I/O lines, or even further by sacrificing the through-hole connections for the I/O lines...

I'm thinking of trying another 9mm wide board design: unfortunately I've found it very tough to make a 9mm design work with all the pins and resistors I'm putting on it... I just run out of space too quickly. All my "version 5" designs were 9mm wide, but they had various problems: on some the "reset" line wasn't connected to the ICSP header, on others the holes for making the external connections may have been too small... I think if I try again I may need to compromise the design features a bit: perhaps omit some of the series resistors on some of the I/O lines. Honestly, though, if version 6 (the 12mm x 10mm board) will fit in my current project I may not bother. Constantly redesigning the same thing does get tiresome. :) I suppose it really would help if I were using design software that provided autorouting, too...

Once I have a design that works well I plan to use it in just about any project of mine that would benefit from a microcontroller... Too much time's been spent in the past trying to make electronics fit in models...

[macfrank]

Constantly redesigning the same thing does get tiresome. :) I suppose it really would help if I were using design software that provided autorouting, too...

The free version of EagleCAD has a pretty good autorouter. The only limitation of the free version are:

* The usable board area is limited to 100 x 80 mm (4 x 3.2 inches).
* Only two signal layers can be used (Top and Bottom).
* The schematic editor can only create one sheet.

[tetsujin]

Yeah - Eagle is what BatchPCB and Sparkfun recommend... I'm kind of a bit inclined to stick to the software libre options, though, so I might give KiCAD a whirl, or just see what the gEDA PCB auto-router can do...