I am interfacing four keys to AT89C51. I am using pull-up resistor for it. So that pins are high when switch is open & low when switch is pressed.
I took reference from mazidi & made keypad routine in C & works fine for me.
Mazidi gave 4x4 keypad algo & I used it to work for individual switches on four pins.
With every press one variable is incremented.

With few more tweaks I am able to detect single click & if key is kept pressed then input gets incremented fast.

Now I wanted to learn how I can implement difference between single & double click.
keys pressed for short time & keys pressed for long time.

1. Any reference algo for it.
2. When interfacing keys with pull-up resistors, should I decouple that pin with 0.1uF capacitor also.

 

Find a microcontroller that has interrupts on change inputs. For example, the MS9S08QG8 has an 8-pin Keyboard Interrupt module that can respond to both edge triggered and level triggered interrupts.

I would not use capacitor on the input pins for noise or switch bounce reduction. Noise can be eliminated via software.