Hi,
Excluding explanation about security when the MCU is locked for external access (the only MassErase command from BDM is accepted) there are four modes the MCU can be entered to:
Unsecured Normal Single Chip mode
Unsecured Special Single Chip mode.
Secured Normal Single Chip mode.
Secured Special Single Chip mode.
This is simple processor from modes point of view (no external bus, no emulation mode as it was for older devices) The MCU enters into its mode during reset sequence on the basis of MODC pin which is copied into MODE register. (1.10.2 Chip Configuration Modes)
Simly you can start MCU in the special mode keeping MODC pin at 0 during reset. This mode stops execution of the code after reset, enters into the BDM mode (so called Special single chip mode) and waits for BDM commands. ( used for debugging, debug interface is connected to a MODC pin to start MCU in this mode)
There is the only one transition of the mode possible by writing a value to the bit MODC in the MODE register as describedin the chapter 4.3.2.1 Mode Register (MODE).
It is worth to mention that some registers are writeable anytime in a special (debug) mode and the only once in the normal mode (MODC pin is connected to log. 1 during reset). This is one source of issues the ctms make. They play a game, during debugging, with some register/s which can be written anytime and everything works. Then they start the MCU in normal mode and suddenly it is not working because the same register/s is write once only in the normal mode.
Best regards,
Ladislav
