Use the standard Serial Wire Debug (SWD) Arm® CoreSight™ Debug Access Port (DAP) protocol to enter debug interface mode.
Before the external debugger can access the CPU, it must first request the device to power up and make sure that the appropriate power domains are powered up. This is managed using the built-in CxxxPWRUPREQ and CxxxPWRUPACK features found in the DAP. If the debugger requests the debug domain or the complete system to power up, the device stays in debug interface mode.