Many integrated circuits include a test access port that may be used to verify the proper operation of the integrated circuit and the proper operation of the system that contains the integrated circuit, including the printed circuit board on which the integrated circuit is mounted. A test access port provides a test clock (TCK) signal, a test mode select (TMS) signal, a test data input (TDI) signal, and a test data output (TDO) signal. The TDI signal may be used to serially shift a bit of data into the integrated circuit each rising edge of the TCK signal and the TDO signal may be used to serially shift a bit of data out of the integrated circuit each falling edge of the TCK signal. A test access port may be implemented according to the IEEE 1149.1 standard and derived standards, such as IEEE 1532 and IEEE 1149.6.
An operating mode for the test access port may determine the destination for data shifted into the integrated circuit at the TDI signal and the source for data shifted out of the integrated circuit at the TDO signal. The operating mode of the test access port may be specified by one of sixteen states of a state vector of a finite state machine. Each state of the finite state machine transitions to one of two possible next states on each rising edge of the TCK signal, with the next state for a current state of the finite state machine determined by the value of the TMS signal at the rising edge of the TCK signal. Thus, the TMS signal may be used to control the operating mode of the test access port.
While the test access port is very useful for determining the cause of improper operation for many aspects of an integrated circuit, the test access port may have limited usefulness for determining the cause of improper operation related to the test access port itself. For example, electrical noise may cause the finite state machine of the test access port to make improper transitions between states, such that the operating mode of the test access port cannot be properly controlled by the TMS signal. Determining the cause of improper operation may be time-consuming and expensive when the operating mode of the test access port cannot be properly controlled due to such electrical noise. The present invention may address one or more of the above issues.