Prepaid energy meter

uses an 8051 and 89c2051.8051 as main microcontroller.89c2051 as smartcard and in it maximum units to be used is programmed.this data is given to 8051.8051 is connected to a digital energy meter.8051 is programmed to decrement the maximum units as aresponse to the information from digital energy meter.There is a buzzer to warn the user.When maximum use is made ,relay is cut off.Then we have to reset both microcontollers using push on switches