PXS20RM Datasheet, PDF (132/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Device Boot Modes
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R 0 0 0 0 0 0 0 VLE
BOOT_ID
W
RES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
ET:
= Reserved
Figure 5-2. Reset Configuration Half Word (RCHW)
Once the device detects that it needs to boot from Flash, and finds a valid BOOT_ID, it will boot from the
application start address at offset 0x04 within the boot sector.
5.3.3 Boot and alternate boot
Some applications require an alternate boot sector so that the main boot can be erased and reprogrammed
in the field.
When an alternate boot is needed, the user can create two bootable sectors; the lowest sector shall be the
main boot sector and the highest shall be the alternate boot sector. The alternate boot sector does not need
to be consecutive to the main boot sector.
This scheme ensures that there is always one active boot sector by erasing one of the boot sectors only:
• Sector is activated (that is, program a valid BOOT_ID instead of 0xFF as initially programmed).
• Sector is deactivated writing to 0 some of the bits BOOT-ID bit field (bit 1 and/or bit 3, and/or bit
4, and/or bit 6).
5.4 Device behavior by boot mode
The following describes which security related device features are available in the device boot modes.
5.4.1 Single Chip Mode — Unsecured
In Normal Single Chip Mode, the system boots from the flash memory main array. The device will boot
from the first sector which is marked bootable. The system may be configured to enable the external bus.
• Nexus/JTAG available
• Boot from flash memory main array
— Shadow Block available
5.4.2 Single Chip Mode — Secured
In Secured Single Chip Mode, the system boots from the flash memory main array. The device will boot
from the first sector which is marked bootable.
PXS20 Microcontroller Reference Manual, Rev. 1
5-4
Freescale Semiconductor