Authentication signals control the debugging modes. The authentication signals configure the processor so its activity can only be debugged or traced in a certain subset of processor modes and security states. See Authentication signals.
The Cortex-A9 processor only supports halting debug-mode debugging in secure User mode when invasive debugging is enabled by the SPIDEN pin. When SPIDEN is LOW, only monitor mode debugging in secure User mode is available by setting the SDR.SUIDEN bit. That is, when SPIDEN is LOW, the processor is not permitted to enter Halting Debug Mode even if the SDR.SUIDEN bit is set to 1. You can bypass this restriction by setting the external SPIDEN pin HIGH.