Just the solution for AVR applications in which I/O is tight, such as the ATtiny12. This should work well on other kids of controllers that have independently controlled I/O direction registers, such as PIC and 6805 controllers.
This is a solution was devised for those I/O limited 8 pin controllers like the AT90S2323 (though I tested it on an AT90S1200A).
In principle, the circuit senses three states of the pushbuttons: Open (no button down), Pulled Up, and Pulled Down. The process is this: Pin A (PORTB bit 4)driven high through a resistor and the other end of the resistor is looked, at Pin B (PORTB bit 4) to see if it is being pulled high, low, or not being driven up a button at all. Then Pin B is driven and the other end of the resistor (Pin A) is looked at.
In the circuit, a 39k resistor is placed across the two I/O pins to allow each pin to drive the other through the resistor. The two pair of buttons connected identically, except that one pair of buttons is connected to pin A (PORTB bit 3) and the other pair of buttons is connected Pin B (PORTB bit 4). In each pair, one button is connected to VCC through a 10k resistor and the other button connects to ground through a 10k resistor.
For more detail: Decoding 4 buttons with two I/O’s on AVR using ATtiny12 microcontroller
