"I think if i use a calculator chip, then i will have to understand EVERYTHING in it. Or what do you think? "

I think it is impossible. Almost all the calculators use ASICs and in most low cost ones the silicon die is bonded directly to the PCB - it just looks a black round piece of tar.

Going by the replies that have been posted I think one can safely conclude that there are NO ready made calculator chips that you can order out ; comes with a data sheet; and can interface with 8052 type MCUs. Bad luck.

You can get back to your coding for the talking calculator. ( And just for your information since you said you want to go commercial with such a product : a few months back I bought a "Talking computer" from Taiwan . This was a toy and had the standard qwerty keyboard and used to talk back whatever was typed on it including the math operators like plus, minus etc. OK it was fun - but only for a while. Two days later even my son wouldn't touch it with a barge pole - he now simply wants to get at the guy inside the unit who is making all the noise !)

Raghu