4-bit µP's A/D converter decodes keypad's inputs

Fig 1 shows a Mitsubishi M50927-XXXSP/FP 4-bit microcontroller decoding a 4×4 keypad using only four digital I/O lines instead of eight. Saving four of a 4-bit µC's precious I/O lines couild be significant. The 4-bit µC's ADC provides the key to the savings. The µC's output lines, F1 through F4, energize the keypad's column lines one at a time via 74HC4049 buffer/ driver. If a user presses a key, the µC's ADC reads an analog voltage coresponding to the row of the pressed key.

Link Source:http://www.edn.com/archives/1994/101394/21di6.htm