Thinking about a keypad - a Big Keypad! 16 x 16.
Use SPI to scan it quick after an interrupt.
Write 1 Row low, read the columns.
Repeat 15 times.
Not sure what to do with 256 keys ...
Have a code sequence in mind, haven't written it out yet tho. Am sure there's a way to intersperse setting the rows while reading the columns to minimize the number of SPI.transfers.
Anyway, back to assembling Bobuino2's.
[edit - resized]
Hmm, bit bigger than I expected. Will resize in a bit.
I think you're a little crazy! 
Good luck with the job.
After soldering thru hole headers all night, I'm thinking all surface mount would be good for this project!
185 x 140mm board is not inexpensive, $80 for 5 bare boards.
Sure beats hand wiring 256 switches tho. I guess it wouldn't be too bad wire wrapping up during 2 hour 0-0 score World Cup games.
surface mount
For the switches?
Would they be able to be run trough a reflow oven and not effect MTBF?
Would be very very nice to have both options so you could use either side.
Switches might be better on the opposite side of the board for some applications. (The side opposite to where the ICs are mounted on.) this would help to keep the height profile down.
Which would help in affixing a top label that has each switch name on it.
5x9, (45) switches is the most I have ever made.
Maybe a PCB could be designed that allows one to break it into different sizes? i.e. 4x8, 5x8, 6x8 and finally 8x8 or other sizes.
Mounting holes are important to keep flexibility down.
0-9, A-Z, 4 arrow keys, and some function keys, probably means 5x8 or 6x8 might be the lower limit.
CrossRoads:
Thinking about a keypad - a Big Keypad! 16 x 16.
You're unlikely to lose that down the back of the sofa!
Offer them in, "solder your own" kit form.
I've redone as 20cm x20cm with space under the buttons for labels, and surface mount logic.
7.9" x 7.9", stencil would cost $85! So I think a 1" x 8" stencil for the top, and the rest thru hole.
The lines are there to line up the parts placement, will be deleted before board ordering.
Use inexpensive tactile switches like these:
http://www.dipmicro.com/store/MJTP1230
http://www.dipmicro.com/store/MJTP1230A
http://www.dipmicro.com/store/TL1105AF250Q
Gonna make up a symbol, see if this part would fit as well for low profile.
Looks like TLK3315 4.5mm will fit on 6mm pads, leaves another 1.5 3 mm for marking.
Suppose it would be pretty easy to make this 4x16, 8x16, 12x16, or full size, 16x16 now that it's layed out.
TLK3315 would lend itself to easier/better labelling.
.55 mm height. Hard to believe.
Yep, pretty small.
Still, seems big compared to 0805 parts!
Seems like that would get in the way of the soldering iron.
I have a pair of prescription 3x reading glasses so I can solder up close and see things clearly.
