And in their PORT value, we set them all as low, so as soon as we change their DDR bit to 1 they become output with value LOW. These are kept at High Impedance State (AKA input), in high z state (z= impedance) state these pins are neither HIGH nor LOW they are in TRI-STATE. The columns C0 to C3 are also connected to MCUs i/o line. They are made an input by setting the proper DDR Register in AVR. The rows R0 to R3 are Input to the Microcontroller. But we cannot connect say 10 or 100 keys directly MCUs because it will eat up precious i/o line and MCU to Keypad interface will contain lots of wires. If we attach a single key to MCU, we connect it directly to i/o line. Many applications require a large number of keys connected to a computing system which includes a PC keyboard, Cell Phone keypad, and Calculators. The concept of interfacing a keypad with the ATmega16 is similar to interfacing it with any other microcontroller. The keypad makes an application more users interactive. The keypad is the most widely used input device to provide input from the outside world to the microcontroller.
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