You could go to any discount store and buy a ready made calculator chip. Just buy the lowest cost calculator you can find and open it up. Inside you will find what is the main problem (or advantage depending upon your point of view) of todays standalone calculator chips. They provide the complete function.....connection from keypad....connection to LCD glass. This type of interface makes these chips nice for making a calculator but almost useless for trying to connect directly to an 8051 type microcontroller. Now I would ask, why when you can buy the low cost calculator readymade for a couple of dollars (or what ever currency you know) why would you consider trying to make another calculator??

Michael Karas