гляньте, не запаролен ли у Вас Secure Block.
Смотрите в даташите пункт 3.2.11:
3.2.11 Security
The F281x and C281x support high levels of security to protect the user firmware from being
reverse-engineered. The security features a 128-bit password (hardcoded for 16 wait states), which the user
programs into the flash. One code security module (CSM) is used to protect the flash/ROM/OTP and the L0/L1
SARAM blocks. The security feature prevents unauthorized users from examining the memory contents via
the JTAG port, executing code from external memory or trying to boot-load some undesirable software that
would export the secure memory contents. To enable access to the secure blocks, the user must write the
correct 128-bit ”KEY” value, which matches the value stored in the password locations within the Flash/ROM.
