Electronic circuit comprising a secret sub-module

A sub-module of a digital circuit contain secret elements such as keys and algorithms. These elements need to be kept secret all along the product life. In particular testing in a critical step, since conventional techniques include structural scan test, which allows downloading of all the internal registers of the circuit and functional application like tests in which the sub-module receives stimuli such as it receives during operation, and outputs respective signal which reveal information on the function of the sub-module by linking the expected output signals to the input signals. The sub-module assembly consist of a sub-module for performing a function and comprising at least one scan chain. A built-in self test circuit including a pattern generator was adapted in test mode.