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Method and circuit for manipulation-proof devaluation of EEPROMS

  • US 5,001,332 A
  • Filed: 12/19/1988
  • Issued: 03/19/1991
  • Est. Priority Date: 12/17/1987
  • Status: Expired due to Term
First Claim
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1. Method for devaluation of a monolithically integrable electronic circuit of a debit card, including at least one address and control logic circuit and a nonvolatile memory having a zone with memory locations provided for storing updated devaluation status of the debit card, at least part of the memory being electrically erasable and all of the memory locations of the zone of the nonvolatile memory can be read out and written upon bit by bit, which comprises dividing the zone of the nonvolatile memory into partial zones each having a different value in the form of a multi-stage counter, permitting a simultaneous erasure of the memory cells only for all of the memory cells of one partial zone of a given value, and permitting erasure of each partial zone only after a transfer bit has been written into a previously unwritten memory cell of the partial zone of next-higher value, monitoring writing-in with a logic circuit, and permitting an erasure of the partial zone of the highest value only under special conditions.

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