Hello Prahlad,

Eric's suggestion seems to be practical and helps in the long run, which is what I mostly do.

You could store your acces codes in the flash, I am doing that for storing calibration data, to avoid the customer copying the eeprom and upgrading from a lower end product to a higher end product.In this instant I am using the P89C669 and the IAP calls.In this chip you can store these values above the 64K flash area.

Another idea, is to implement the following device in your products: DS2502 this is a one wire add only memory, you can get more details from here:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2924

I found this chip implemented in an inkjet printer's ink catridge ( used in the advertisement industry), these are expensive catridges, (around Rs.9000/= /190US$)to prevent the buyer of the catridge from refilling the catridge.

Regards,
John.