I found this collection of multiplexing strategies for keyboard matrices, as part of a series for building your own synthesizer: <http://www.openmusiclabs.com/learning/digital/input-matrix-s...>

Forgot to note, if you're really short of GPIO, you could also use two 8-line to 1-line data selector chip, e.g. 74151, so you'd have a 74138 to select one of 8 rows, and two 74138 which select a column each, and two further input pins to read from the result. That would reduce GPIO down to 8 (6 out, 2 in).

And just in case you're really really short of GPIO, you can use a 74595 shift register to feed the column and row selection, so you'd just need clock and data outputs.

You could also use 74251 8-to-1 multiplexer as that has an output enable, so you could use your remaining 2 outputs from the shift register to enable one of the two multiplexer chips. If you're not latching the outputs on the shift register, then you'd want a resistor between the two outputs so they don't fight.

So then, the ultimate reduction in GPIO would be to 3 lines - clock, data, in.

Obviously, for each of these, as you reduce the number of IO pins, you increase the time required to scan the matrix.

74595s are incredibly useful, compatible with SPI interfaces, and completely forgotten by today's engineers.

I have a current project where I needed to drive a large number of CS lines on a SPI bus. Others suggested I/O expanders and even - good lord - an entire M0 controller to handle it. I solved it with a bitbanged chain of 74HC595s, under $1 in BOM cost.