Microchip ATSAM D21 (Cortex-M0+) uses NVM fuse read protection. SAM D21 decryption bypasses the protection to recover flash firmware.

ATSAM D21 Protection:
- NVMCTRL fuse bits lock flash.
- Reading locked flash returns zeros.
- Debug access (SWD/JTAG) is disabled.
- Changing fuses requires chip erase.

SAM D21 Decryption Methods:
1. Voltage Glitching: glitch VDD during fuse check.
2. Clock Glitching: inject faults into NVM controller.
3. Optical Decapsulation: remove package, read die.
4. Side-Channel: power analysis on NVM reads.

Recovered firmware can be used for product backup, repair, or PCB cloning. SAM D21 is popular in IoT, wearables, and industrial controls.