A 1-Wire port expander can be used to sense 8 push-buttons with only 2 wires. It can be powered via a strong pullup on the data line, so a separate power line isn’t needed.
For applications that require less than a keyboard or numeric keypad, this simple circuit monitors multiple pushbutton switches using a single multi-channel 1-Wire addressable switch (DS2408), and a few external components.
This is a fun exercise, but we’d steer clear of it in a design for a number of technical and practical reasons. Our personal hunch is that 1-Wire’s days are numbered. Maxim has discontinued many devices, and third-parties can’t release new ones due to the 1-Wire license. It seems like the most available 1-Wire parts are the EEPROMs and IO expanders, stuff they need to keep selling to avoid the wrath of large, long-term customers.
I agree that 1-wire’s days look numbered. But the same port expander idea can be applied with SPI and I2C devices from microchip and others.
Dare I suggest that there’s plenty of I2C/SPI equivalents out there which have more features and are less “proprietary”. MCP23017 being a good example.