Patent ID: 7936875
Filing Date: 2011-05-03
Classification: G01R,H04L

Abstract:
1. A method for protecting a digital quantity stored in a microcontroller comprising a JTAG interface, the method comprising: making said digital quantity stored in the microcontroller comprising the JTAG interface dependent on a value of a parameter, wherein at least a first value of the parameter is stored in non-volatile memory in the microcontroller; in a first operating mode of the JTAG interface when signals are absent at an input of the JTAG interface, providing a first value of the digital quantity by combining the digital quantity with the first value of the parameter; and in a second operating mode of the JTAG interface, in response to signals being present at the input of the JTAG interface, activating protection of a correct value of the digital quantity by making the correct value inaccessible by combining the digital quantity with a second value of the parameter, wherein: the second value of the parameter is different from the first value of the parameter, the first value of the parameter is set during manufacturing of the microcontroller, and the first value of the parameter is a parameter for generating a key used by a cryptographic function of the microcontroller.