Keypad intergration questions

[kam]

Hi all!

I have a need to interface to a keypad. From what I know (so far!), I can use 8 pins for a 4x4 matrix keypad etc....my question is, what if my keypad was 6x6? More pins? Different approach? Come to the realization that 6x6 is WAYYYYYY to much and that they should really want the 4x4?

Thanks!

~Kam (^8*

[Ttelmah]

It all depends on how many pins your processor has available. Yes, you can just scan 6*6. Larger keyboards like this are commonly scanned just this way using a micro. However when the chip has to do something else as well, the typical solution, is to add an external multiplexer/demultiplexer to keep the pin count down. For instance, if you use a 3 to 8 line decoder, and drive this from three lines on the chip, you can scan a 8*6 keypad, using just 9 lines (six inputs, and three to drive the 'column select' using the decoder). Add a eight input multiplexer, and you can handle a 8*8 keyboard matrix, using just seven lines (three to turn on the column, three to select the row, and one input bit).
If the keyboard is a custom matrix, then consider designing the layout to simplify the pins needed. For example, you can scan a 4*8 matrix, using four output lines, one input line, and an eight input multiplexer, to give up to 32 keys (enough for a simple 'alpha' keypad, with a few extra control keys), on a single 8 pin port.

Best Wishes

[treitmey]

You could also use a keypad decoder chip. Then interface to that chip with for example 2 pins and I2C.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/3414