Seeking 4x3 keypad for Atmel UC3 microprocessor
The circuit I use has the MM74C922 4x4 matrix encoder at its heart. The advantages of using it are that it provides an interrupt for keypress notification plus it handles debouncing the keys.



DA connects to an interrupt pin so that the MCU can be notified when a key has been pressed. For 4x3 operation, leave Y4 disconnected (since the Yx pins are sensing).

Note however that the MM74C92x is obsolete, and supplies will dry up over time. You should look for a more recent part such as the TCA8418 or MAX7360 which, although not drop-in replacements, do offer additional capabilities which may remove some of the burden from the MCU.