Some pitfalls and thought on SparkFun’s Button Pad SPI. I’m using it with Arduino Duemilanova.

The Button Pad SPI is NOT standard SPI. You cannot use it with Arduino’s (Atmega’s) built-in SPI. You have to write your own code to handle the communications.

• Yes, it’s possible to hook multiple boards and read all buttons statuses with only one “SPI” line. This however might be bad idea, see below.
• The manual confuses you with the feedback-line. That’s actually MOSI from the last board. So hook that to input pin on Arduino and read the button statuses from that.
• You cannot hook the boards parallel to the SPI line (using separate CS lines) since the boards don’t respect the CS. They will receive data even if the CS line is down.
• If you want to control the boards separately you must have separate CS and SCK lines. MISO can be common.
• The boards can loose their multi-board configuration setting and think they are in one-board-environment. This sucks. Hard. You have to reprogram the boards separately, the configuration works only for the board where the “SPI” is connected to. Of course this isn’t SparkFun’s problem: you’re causing this by for example when your Arduino boots or there’s some other clitches on the “SPI”-line. I think it should be more difficult to change the setting, for example only when some dip-switch or something is connected/unconnected..
• You cannot press more than 2 buttons on a column. However, in rows you can press and receive all 4 buttons.
• You cannot align the boards in 2*2 shape with all having same orientation (column/row).
• It’s not 24-bit color scheme, or at least the color values are not linear to what you see. The color value has exponential-like effect to the actual led brightness.
• It’s still a great product for the price.
• It looks very very sweet. In dark it looks like a candy!
• On user inteface side these boards work just like they are supposed to work. Unlimited possibilities in the software that you write for them. Be creative, be fun!