Cortex-A8 R2P2.pdf - ARM Information Center

DebugTable 12-57 Authentication signal restrictions (continued)SPIDEN DBGEN a SPNIDEN NIDENSecureinvasivedebugpermittedNonsecureinvasivedebugpermittedSecurenoninvasivedebugpermittedNonsecurenoninvasivedebugpermitted0 1 1 0 No Yes Yes Yes0 1 1 1 No Yes Yes Yes1 0 0 0 No No No No1 0 0 1 No No Yes Yes1 0 1 0 No No No No1 0 1 1 No No Yes Yes1 1 0 0 Yes Yes Yes Yes1 1 0 1 Yes Yes Yes Yes1 1 1 0 Yes Yes Yes Yes1 1 1 1 Yes Yes Yes Yesa. When DBGEN is LOW, the processor behaves as if DSCR[15:14] equals b00 with the exception that halting debug eventsare ignored when this signal is LOW.Changing the authentication signalsThe NIDEN, DBGEN, SPIDEN, and SPNIDEN input signals are either tied off tosome fixed value or controlled by some external device.If software running on the processor has control over an external device that drives theauthentication signals, it must make the change using a safe sequence:1. Execute an implementation-specific sequence of instructions to change the signalvalue. For example, this might be a single STR instruction that writes certainvalue to a control register in a system peripheral.2. If step 1 involves any memory operation, issue a Data Synchronization Barrier(DSB).3. Poll the DSCR or Authentication Status Register to check whether the processorhas already detected the changed value of these signals. This is required becausethe processor might not see the signal change until several cycles after the DSBcompletes.12-92 Copyright © 2006-2008 ARM Limited. All rights reserved. ARM DDI 0344E