Standby is the least invasive OS energy saving state because it only implies that the core is unavailable. It does not clear any of the debug settings. For this case, if DBGNOCLKSTOP is HIGH, the processor guarantees the following:
If the processor is in standby and a halting debug event occurs, the processor:
retires the Wait-For-Interrupt (WFI) instruction
enters debug state.
The processor responds to APB accesses as if it was not in standby.
If you implement the CoreSight Debug Access Port (DAP) in your system, ARM recommends that the DAP CDBGPWRUPREQ output is connected to the DBGNOCLKSTOP processor input.