16.5.1 Boot Configuration Word

The Boot Configuration Word allows several customizations of the Boot Sequence:
  • To configure the IO Set where the external memories used to boot are connected (see Hardware and Software Constraints for a description of the IO sets)
  • To disable the boot on selected memories
  • To configure the UART port used as a terminal console
  • To configure the JTAG pins used for debug

See the section Boot Configuration Word for a detailed description of all the bitfields in this word.

By default, the value of this word is 0x0.

For MRL A and B parts, the ROM code does not try to detect a valid bootable software in any external memory, and runs directly the SAM-BA monitor. See the figure NVM Bootloader Program Description for MRL A and MRL B Parts.

For MRL C parts, the ROM code only tries to boot on SDMMC1 and SDMMC0 memory interfaces and then run the SAM-BA monitor. See the figure NVM Bootloader Program Description for MRL C Parts.

During prototyping phases, the value of this fuse word can be overridden by the content of a backup register. The conditions to enable this feature are as follows:
  • The fuse bit DISABLE_BSCR must not be set (default value).
  • The Boot Sequence Controller Configuration register (BSC_CR) must have the BUREG_VALID bit set and indicate in BUREG_INDEX which register has to be used.

Using BUREG allows the user to test several boot configuration options, including Secure Boot Mode, without burning fuses.

Note: VDDBU must be connected in order to benefit from this feature. However, in production, it is highly recommended to disable this feature and to write the boot configuration in fuses.
Figure 16-2. Boot Configuration Loading