System and method for protecting contents of microcontroller memory by providing scrambled data in response to an unauthorized read access without alteration of the memory contents
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United States of America Patent
Stats
-
Aug 29, 1995
Issued Date -
N/A
app pub date -
Mar 7, 1994
filing date -
Nov 12, 1991
priority date (Note) -
Expired
status (Latency Note)
Importance
Abstract
A microcontroller fabricated on a semiconductor chip has an on-chip EPROM program memory. The microcontroller is selectively configurable to operate in any one of a plurality of predetermined operating modes, including at least one secure microcontroller mode. A plurality of EPROM configuration fuses used for configuring the microcontroller and protecting its program memory from read, verify or write through any instruction initiated from other than a predetermined secure area of the chip, are mapped into the on-chip EPROM program memory as bits in respective address locations thereof. The value of a bit representing any one of said fuses is effective to determine the condition of the respective fuse. That condition is observed by reading the value of the respective bit for that fuse stored in the EPROM program memory. The chip security is enabled by configuring the microcontroller in a code protected mode by programming the bits representing the desired fuses in the EPROM program memory to effectively blow or erase each fuse according to the desired configuration.
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