Patent ID: 8707443

Claim:
A circuit operable in a normal operating mode and a test mode, the circuit comprising: a testable circuit; a privileged information supply circuit for providing privileged information, selectively coupled to the testable circuit; a test access circuit coupled to the testable circuit; a test control circuit coupled to the test access circuit and configured to control switching of the test access circuit to the test mode, characterized by a shadow information supply circuit for providing shadow information that is a substitute for the privileged information; and a multiplex circuit coupled between the testable circuit and the privileged information supply circuit and the shadow information supply circuit, the multiplex circuit having a control input coupled to the test control circuit, the multiplex circuit being configured to couple the privileged information supply circuit to the testable circuit for access to privileged information or instead, selectively in response to a control signal from the test control circuit indicating that the test access circuit is enabled to capture test response signals from the testable circuit in the test mode, to couple the shadow information supply circuit to the testable circuit so that the privileged information and shadow information from the shadow information supply circuit are supplied to the testable circuit in the normal operating mode and the test mode, respectively.