What is a Keypad?

A keypad is a set of buttons or keys bearing digits, symbols and/or alphabetical letters placed in order on a pad, which can be used as an efficient input device. A keypad may be purely numeric, as that found on a calculator or a digital door lock, or alphanumeric as those used on cellular phones.

Explanation for Keypad

Aside from the row of number keys found on the upper portion of a computer keyboard, a separate numerical pad is also located on the right side for efficient data entry. For more compact computers such as laptops and notebooks, external plug-in keypads may be purchased separately.

Phone keypads are typically alphanumeric, allowing the user to enter names and text messages. They also provide an easier way of remembering phone numbers. For instance, 1-800-DELIVER is a way of marketing a food chain delivery service, but it's easier to remember than the actual number customers would call (1-800-335-4837).

How many I/0 interface pins between keypad (4x4) and PIC18f4520?
A standard Microcontroller Keypad (4x4) has a 8 I/0 interface pins.

In the example below, RB0 to R07 is connected to the 8 I/O pins of the Keypad (4x4)

The standard PIC18f4520 Microcontroller has 36 I/O pins that are grouped into 5 ports respectively.
Due to this, It should require 8 pins to connect the 4x4 Keypad to make it work.

How to detect if the key is pressed?

E.g. Of flowchart when things are input.
When a key depression has been detected and an interrupt generated, the MCU will need to determine which of the keys is being pressed.

E.g. Initial a row high, and scan the 4 columns to see if any are high, if one is, then you have found a pressed key.

How to identify which of the 16 keys were depressed
(decode the key from the (4x4) matrix)?

The Keys are in a matrix. Thus, the PIC must determine which
row