"I have a problem reading a ibutton on a DS320."

Is it actually an iButton problem, or a problem with the DS320?

"The same code work 100 % on a AT89S8252.
Use P3.4 as data I/O, On the DS320 there is no data on this pin"

What do you mean by "no data"

With no iButton connected, can you see the 1-Wire Reset pulse?
If not, then your problem lies with the DS320 and you'll have to lay the datasheets side-by-side to try and spot the config differences which could cause this!