ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Recommended External Debug Interface
Changing the authentication signals
In v6.1 Debug and v7 Debug, the NIDEN, DBGEN, SPIDEN, and SPNIDEN authentication signals can
be controlled dynamically, meaning that they might change while the processor is running, or while the
processor is in Debug state.
Note
In v6 Debug DBGEN is a static signal and can be changed only while the processor is in reset.
Normally, these signals are driven by the system, meaning that they are driven by a peripheral connected to
the ARM processor. If the software running on the ARM processor has to change any of these signals it must
follow this procedure:
1. Execute an implementation specific sequence of instructions to change the signal value. For example,
this might be an instruction to write a value to a control register in a system peripheral.
2. If step 1 involves any memory operation, perform a Data Synchronization Barrier (DSB).
3. Poll the debug registers to check the signal values seen by the processor. This is required because the
processor might not see the signal change until several cycles after the DSB completes.
4. Perform an Instruction Synchronization Barrier (ISB), exception entry or exception return.
The software cannot perform debug or analysis operations that rely on the new value until this procedure
has been completed. The same rules apply for instructions executed through the DBGITR while in Debug
state. The processor view of the authentication signals can be polled through DBGDSCR[17:16] and, in v7
Debug, the DBGAUTHSTATUS register.
Note
Exceptionally, the processor might be in Debug state even though the mode, security state and authentication
signal settings are such that, in Non-debug state, debug events would be ignored. This can occur because:
it is UNPREDICTABLE whether the behavior of debug events that are generated between a change in
the authentication signals and the next Instruction Synchronization Barrier, exception entry or
exception return follow the behavior of the old or new settings
it is possible to change the authentication signals while the processor is in Debug state.
See also Generation of debug events on page C3-40 and Altering CPSR privileged bits in Debug state on
page C5-14.
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