7.9 Memory Section Access from CPU and UPDI on Locked Device

The device can be locked so that the memories cannot be read using the UPDI. The locking protects both the Flash (all Boot, Application Code, and Application Date sections), SRAM, and the EEPROM including the FUSE data. This prevents successful reading of application data or code using the debugger interface. Regular memory access from within the application is still enabled.

The device is locked by writing a non-valid key to the LOCKBIT bit field in FUSE.LOCKBIT.

Table 7-5. Memory Access Unlocked (FUSE.LOCKBIT Valid Key)(1)
Memory Section CPU Access UPDI Access
Read Write Read Write
SRAM Yes Yes Yes Yes
Registers Yes Yes Yes Yes
Flash Yes Yes Yes Yes
EEPROM Yes Yes Yes Yes
USERROW Yes Yes Yes Yes
SIGROW Yes No Yes No
Other fuses Yes No Yes Yes
Table 7-6. Memory Access Locked (FUSE.LOCKBIT Invalid Key)(1)
Memory Section CPU Access UPDI Access
Read Write Read Write
SRAM Yes Yes No No
Registers Yes Yes No No
Flash Yes Yes No No
EEPROM Yes Yes No No
USERROW Yes Yes No Yes(2)
SIGROW Yes No No No
Other fuses Yes No No No
Note:
  1. Read operations marked No in the tables may appear to be successful, but the data are not valid. Hence, any attempt of code validation through the UPDI will fail on these memory sections.
  2. In the Locked mode, the USERROW can be written using the Fuse Write command, but the current USERROW values cannot be read out.
Important: The only way to unlock a device is through a CHIPERASE. No application data are retained.