ARM Cortex-A15 MPCore Processor Technical Reference Manual

DebugIf 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 signal value.For example, this might be a single STR instruction that writes certain value to a controlregister in a system peripheral.2. If step 1 involves any memory operation, issue a DSB instruction.3. Poll the DBGDSCR or Authentication Status Register to check whether the processor hasalready detected the changed value of these signals. This is required because the systemmight not issue the signal change to the processor until several cycles after the DSBinstruction completes.4. Issue an ISB instruction exception entry or exception return.The software cannot perform debug or analysis operations that depend on the new value of theauthentication signals until this procedure is complete. The same rules apply when the debuggerhas control of the processor through the Instruction Transfer Register, DBGITR, while in debugstate. The relevant combinations of the DBGEN, NIDEN, SPIDEN, and SPNIDEN values canbe determined by polling DSCR[17:16], DSCR[15:14], or the Authentication Status Register.ARM DDI 0438I Copyright © 2011-2013 ARM. All rights reserved. 10-37ID062913Non-Confidential