When I set port B as an input port, I used only one pin said pin 18 for my input device. The other pins such as 19-25 hadn't tied high or low (i.e. floating). As my system depended upon the reading in port B. My system is very stable from just turn on to about 45 mins. After 45 mins, the system became unstable. Should I need to tie the unused pins to high or low? Can I use Intel Basic-52 Language to check the pin 18 is high or low with ignoring the other pins in port B?

XXXXXXX0

with some calculations -> 0 -> low

XXXXXXX1

with some calculations -> 1 -> high

The worst case (1) (tied high):

11111110 -> FE -> low

11111111 -> FF -> high

The worst case (2) (tied low):

00000000 -> 0 -> low

00000001 -> 1 -> high