PXS20RM Datasheet, PDF (130/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Device Boot Modes
MMU page must be properly aligned , which is achieved by setting the 12 LSB's of the reset vector address
stored into this entry to 0.
The MMU VLE bit of this TLB entry is set depending on the status of the VLE bit within the RCHW. This
means that the most efficient place to put the application code is immediately after the boot sector.
The 4 KB block provides sufficient space to
• Add MMU entries for the remaining program space, SRAM and peripherals
• Perform standard system initialization tasks (initialize the SRAM, setup stack, copy constant data)
• Transfer execution to RAM, re-define the flash memory MMU entry and transfer execution back
to flash memory.
5.2.1.1 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, 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).
5.3 Boot-sector search
5.3.1 Potential boot sectors
As shown in Figure 5-1 in single chip mode the device will search several locations for a valid boot ID.
The lowest sector which starts with a valid boot ID will be used to boot the device.
The Flash locations 0x0000_0000, 0x0000_4000, 0x0001_0000, 0x0001_C000, 0x0002_0000 and
0x0003_0000 will be searched.
PXS20 Microcontroller Reference Manual, Rev. 1
5-2
Freescale Semiconductor