The present invention provides a secure JTAG interface to an application-specific integrated circuit (ASIC). In the preferred embodiment the invention operates through the combined efforts of a Security Module (SM) comprising a state machine that controls the security modes for the ASIC, and a Test Control Module (TCM) which contains the JTAG interface. The TCM operates in either a restricted mode or an unrestricted mode, depending on the state of the SM state machine. In a restricted mode, only limited access to memory content is permitted. In an unrestricted mode, full access to memory content is permitted.