12.6.2 CPU Reset Extension

“CPU Reset extension” refers to the extension of the Reset phase of the CPU core after the external Reset is released. This ensures that the CPU is not executing code at start-up while a debugger is connects to the system. The debugger is detected on a RESET release event when SWCLK is low. At start-up, SWCLK is internally pulled up to avoid false detection of a debugger if the SWCLK pin is left unconnected. When the CPU is held in the Reset extension phase, the CPU Reset Extension bit of the Status A register (STATUSA.CRSTEXT) is set. To release the CPU, write a '1' to STATUSA.CRSTEXT. STATUSA.CRSTEXT will then be set to '0'. Writing a '0' to STATUSA.CRSTEXT has no effect. For security reasons, it is not possible to release the CPU Reset extension when the device is protected by the NVMCTRL security bit (See NVMCTRL.CTRLB.CMD's Set Security Bit (SSB) command.). Trying to do so sets the Protection Error bit (PERR) of the Status A register (STATUSA.PERR).