4 Boot Configuration
The boot sequence is controlled using a Boot Configuration Word (BCW) either in the Fuse area or in the backup registers BUREGx. By default, the value of the BCW is 0x0.
The following figure shows the Boot Configuration loading sequence.
The Boot Configuration Word allows several types of customization of the boot sequence:
- Configuration of the IO set where the external memories used to boot are connected
- Disable the boot on selected memories
- Configuration of the UART port used as a terminal console
- Configuration of the JTAG pins used for debug
During the prototyping phase, it is recommended to configure the boot sequence by using the Boot Configuration Word stored in one of the backup registers BUREGx.
There are four backup registers. The user can choose the Boot Configuration Word from a specific backup register by setting BUREG_INDEX (BUREG0, BUREG1, BUREG2 and BUREG3) in the Boot Sequence Controller Configuration register (BSC_CR).
Two conditions are necessary to write the Boot Configuration Word to BUREGx:
- The Fuse bit DISABLE_BSCR must not be set (default value).
- BSC_CR.BUREG_VALID must be set and BUREG_INDEX must be configured to indicate which register is used.
- If BSC_CR.BUREG_VALID is not set, the Boot Configuration Word from the Fuse is used.
- If VDDBU power is removed, the Boot Configuration Word is erased from the backup register. The Boot Configuration Word has to be written to backup registers every power cycle.
For the production phase, it is highly recommended to set the DISABLE_BSCR bit in the Boot Configuration Word and write the boot configuration in Fuses.
The internal RC clock should be enabled to write the Boot Configuration Word to Fuse.