7.4 Program and Debug Interface Disable (PDID)

After activating the Program and Debug Interface Disable (PDID), the only way to write to the reprogrammable Flash memory (nonvolatile memory - NVM) is from the Boot Code section of the NVM. Consequently, CHIPERASE or other re-programming attempts through the UPDI will fail. Also, any attempt to read out any NVM content will fail.

Use the following procedure to enable the PDID feature (restrict access to NVM):

  1. Write 0xB452 to the PDI Configuration (PDICFG) fuse:
    • Provide the NVM Protection Active (NVMACT) key by writing 0xB45 to bits PDICFG[15:4] (KEY)
    • Bits PDICFG[3:2] are unused - ensure they are zero
    • Select the Protection Level NVM Access Disabled (NVMACCDIS) by writing 0x2 to PDICFG[1:0] (LEVEL)
  2. Write the Lock Key Bits (KEY) in the LOCK.KEY fuse to LOCKED.
  3. Reset the device.

Once protection level NVMACCDIS is invoked, the following access rules apply:

  • NVM access through UPDI is disabled
  • Updates to the application software can only be performed by code located in the Boot Code section (bootloader)
  • Chip Erase is disabled
  • User Row write access is disabled
  • CRC status will be available
Important: Unlike for locked devices, performing a CHIPERASE through the UPDI interface once the PDID feature is activated is impossible. The only way to alter the NVM content after PDID activation is by executing NVM writes from the Boot Code section (bootloader). The application software must ensure that the bootloader implementation fulfills the security requirements.
Note: After PDID activation, access to NVM is very restricted for external testing. Some testing will be possible, but advanced failure analysis will not be possible.