4x4 Keypad 5 Wire interface.
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Five lines is plenty for some cheap 74' chip I/O solutions. I'll not post a solution now because I don't know what other constraints like board space etc, you may have. But a quick example would be to use half of a '139 to select which column you're polling (consumes 2 IO lines) and then use half of a '153 to select which row you are polling (consumes 2 different IO lines).

While that take only 4 lines and only utilizes half of two inexpensive chips, it does prevent you from doing some keypad polling shortcuts. For example, if connected directly to a micro you could enable all columns and search or any row going active before triggering a row/col polling search (In US Football we'd call this a "Quick-Out"). With this quick circuit (don't try to use the same address lines on both chips or you'll only read a diagonal - hehehe) you may find it cheaper than other specail built chips. I don't know if board space is a concern.

I recommend that you look at the databook on the two chips mentioned above and then look around for a better fit. Often people only need to hear an alternate idea to get started until they find something even better.

By the way, having a spare half '139 and a spare half '153 like device is more likely to find a use than other residual nonbasic gates. At least when drawing them on a schematic... it may not be easy for the PCB layout guy. :-)

I expect someone will soon post a brilliant circuit idea so that's all I'll offer for now. :)

I2C Parallel Port Expanders:
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My PCF8574 Datasheet has a note at the top: "$2.07 @ 500, 9812" which means that in December of 1998 I was quoted a price of $2.07 per chip. Contrast that against less elegant 74' series ranging from $0.18 to $0.45 in like quantities.

I like the PCF8574 but I also can't recommend it as a practical alternative in designs I've done. Its nice, bi-directional ports with wired-or interrupt and a I2C interface. Its chip cost is too expensive for most production designs unless you really get into a bind.

Its champion application I believe was in Xerox copiers where physical interconnections were minimized. But Xerox doesn't design cost-minimal devices either.

I've looked at I2C port exapanders a few times as but I still haven't used it. A keyboard scanner over I2C increases the micro's time commitment for debouncing the keypad. You could wire the interrupt for keytouch to minimize polling until debounce.

The reason its seldom used is simply because there are often more inexpensive solutions.

-Jay C. Box