The erase all function lets the debugger trigger an erase of flash, user information configuration registers (UICR), RAM, all peripheral settings, and also temporarily removes the access port protection.
To trigger an erase all function, the debugger writes to the register ERASEALL. The register ERASEALLSTATUS will read as busy for the duration of the operation. The ERASEALL mechanism completes its tasks by writing UICR.APPROTECT to the Unprotected value, in addition to writing the CPU side CTRLAP.SECUREAPPROTECT.DISABLE and CTRLAP.APPROTECT.DISABLE registers to the value 0x50FA50FA. After the next soft reset, the access port protection is temporarily removed. This temporary unprotection is removed by the next pin reset, power-on reset, brown-out reset, or watchdog timer reset. For more information about access port protection, see Access port protection.
If the debugger performs an erase all function on a slave MCU, the erase sequence will always erase the application MCU first, independently of how the application is protected, before erasing the slave MCU.
Erase all protection
It is possible to prevent the debugger from performing an erase all operation by writing to the UICR.ERASEPROTECT register. Once the register is configured and the device is reset, the CTRL-AP ERASEALL operation is disabled, and all flash write and erase operations are restricted to the firmware. In addition, it is still possible to write/erase from the debugger as long as the UICR.APPROTECT register is not set.
The register ERASEPROTECT.STATUS holds the status for erase protection.