There are more and more systems which make use of electronic memory cards, e.g. "smart" cards, in particular electronic payment systems for providing goods or services such as telephone calls, distributing bank bills, supplying objects, paying parking fees, etc.
In addition to processing circuits of varying degrees of sophistication, a memory card includes a memory which, at present, is a non-volatile memory. The memory may be of the EEPROM type or of the EPROM type. If the memory is an EEPROM, i.e. a non-volatile memory capable of being erased electrically, storing successive balances presents little problem since the old balance can be erased prior to the new balance being recorded.
In an EPROM type of memory, stored data can be erased only by erasing all of the stored data under the effect of ultraviolet radiation. In practice, this means that the recording of data in a card fitted with an EPROM is irreversible. More precisely, each memory location which is initially in a "0" state is capable of being changed irreversibly to the "1" state when written to.
It can readily be deduced from the above that when an EPROM is being used, it is particularly advantageous to provide a procedure for writing successive balances which uses up as small a number of memory locations as possible.
It must also be emphasized that the card must be rechargeable, i.e. not only must it be capable of recording balances corresponding to debits, it must also be capable of recording balances corresponding to the card being recharged, i.e. to credits.
French patent application published under the number 2 591 007 describes a procedure for writing in an EPROM, which procedure is intended to solve this problem and is referred to as a "token-carrier" procedure. However, the technique described in that document is complex and does not provide optimum utilization of the memory capacity.
An object of the invention is to mitigate this drawback by providing a method of recording successive balances in a memory, which method is easy to implement and ensures optimum utilization of the memory capacity.