atmega 8535 pullup and pull down and sensor input

[deepakgupta.rf]

hello everyone
i use atmega 8535 and use port A for sensor input
sensor like door sensor,pir sensor and some time in water controller in water controller use cd 4069ube for signal getting from tank then interfacing from this IC i give directly to avr .there i am getting confused is there any need with avr input like pull up and pull down
please expalin these terms also

 

[alexxx]

Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.

Pull-up resistor - Wikipedia, the free encyclopedia


AVR has internall pull-up resistors, so you don't need to use external resistors for this purpose, except some special occasions.

**broken link removed**
View topic - Internal pull-up advantages? :: AVR Freaks
https://www.edaboard.com/threads/68923/


Also if you are interested about debounce:

AVR Tutorial: Switch Input and Debounce


Hope that helped.