Hey Kevin, glad you are here. I have been thinking about the packaging quite a bit so hope you don't mind if I consolidate some of those thoughts here.
It looks promising, but I think a ftdi chip (USB) might enjoy a larger audience. My feeling is that I initially underestimated the footprint of legacy free machines out there when this project started.
Another option I'm considering recommending for the
DIY PCB version is get an iduino and an extra atmega168 and pcb and do a chip swap AND be left with an iduino to play around with
Or possibly just a kit with the software already loaded (need to wait for more stable software on that one). Pre-Programmed would probably have the largest audience though, and there wouldn't be a "whoops I forgot to spend another $20 on a usb cable" surprise to lay on folks.
The other option that comes up frequently is a shield of some sort, lots of arduino layout boards out there but if there was a board that soldered to the back of a lcd that held the extra components/buttons that an iduino could plug into, that would likely have some appeal.
Last options would be a separate kit (similar to your ftdi adapter) that should cost a lot less than a ftdi cable but have the same pinout. Could also make a rs232 with the same ftdi cable pinout (and a power plug/voltage regulator) so the buyer picks which one they want (assuming they get plenty of warning that they will need one or the other), and it plugs right into the ftdi connector and a usb or serial cable/power adapter.
I've pretty much written off rs232 though, especially since a programming adapter can be improvised with a couple transistors (crib off of serial 1.0).
Buttons, on all my versions I put the buttons in the upper right in the LCD border, so I press down on them, and so there is a left and right and middle. It is a PITA to translate that to a simple PCB though, I wind up making them a bit like making jewelery ( see
this post ) but I have a couple things I still want to try in that department. The less profile this has, the better it will work in front of your instrument cluster where you don't have to go looking for it.
The vehicle pins should be as low as possible. Again, this helps with the profile, so you aren't hiding important warning lights or something with your mpg gauge.
LCD, currently trying out some newhaven display ones from mouser. The consensus is that the blue ones suck. The green ones are much better especially if you crank up the contrast. Also keeping temperature performance in mind. Here is the display number I'm currently trying: NHD-0216K1Z-FL-YBW They have less expensive ones from newhaven with the same specs except they have side and bottom pinouts, the side pinouts are a bust, they interfere too much with the vehicle interface location options.
And FWIW, here are the zener part number: 1N4733A-TR (same as radioshack stocks), might matter.
Also, just curious, The Schottky isn't a bad idea, but where'd you get the idea from? Also FYI, I haven't had any trouble with nothing connected to the reset pin for the chipswap versions (left agrnd and avcc alone as well).
The pinout to the buttons and the LCD is the same, correct?
Sorry if it's long winded
I'm sure you have thought about a lot of this stuff more than I have. But there it is, open for discussion.