10.6.2. Changing the authentication signals

The NIDEN, DBGEN, SPIDEN, and SPNIDEN input signals are either tied off to some fixed value or controlled by some external device.

If software running on the processor has control over an external device that drives the authentication 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 control register 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 has already detected the changed value of these signals. This is required because the system might not issue the signal change to the processor until several cycles after the DSB instruction 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 the authentication signals until this procedure is complete. The same rules apply when the debugger has control of the processor through the Instruction Transfer Register, DBGITR, while in Debug state. The relevant combinations of the DBGEN, NIDEN, SPIDEN, and SPNIDEN values can be determined by polling DSCR[17:16], DSCR[15:14], or the Authentication Status Register.

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