One solution are usage of external shmitt trigger like 74HC14 with RC-circuit at input.
But avr microcontrollers have internal shmitt triggers at inputs.
I measure input hysteresis at one input of my atmega328p and get about 0.15V.
This is lower then 0.4V min of 74HC14.
Are internal shmitt triggers sufficient or no?
Sufficient for what purpose? It totally depends on the circuit and the application.
But avr microcontrollers have internal shmitt triggers at inputs.
No not really, if you look at the specification there is precious little hysteresis especially when compared with a 74xx14.
Are internal shmitt triggers sufficient or no?
I would say no.
Which of course begs the question - why would you want to use hardware debounce when you have a microcontroller? 