PXD20RM Datasheet, PDF (181/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

6.5.3 Single Chip boot mode
In single chip mode the hardware searches flash boot sector for a valid boot ID. As soon the device detects
the first bootable sector, it jumps within this sector and reads the 32-bit word at offset 0x4. This word is
the address where the startup code is located (reset boot vector).
The hardware then sets up a 4 KB MMU entry at this address. The instruction set for this entry is selected
by the VLE bit in the RCHW. Then the device executes this startup code. A user application should have
a valid instruction at the reset boot vector address. The user application should set up the MMU to allow
access to peripherals and memory as required.
If a valid RCHW is not found, the BAM code is executed. In this case BAM moves the PXD20 into static
mode.
6.5.3.1 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 allows to ensure that there is always one active boot sector by erasing one of the boot sectors
only:
• Sector shall be activated (that is, program a valid BOOT_ID instead of 0xFF as initially
programmed).
• Sector shall be deactivated writing to 0 some of the bits BOOT-ID bit field (bit1 and/or bit3, and/or
bit4, and/or bit6).
6.5.4 Boot through BAM
6.5.4.1 Executing BAM
Single Chip mode is managed by hardware and BAM does not participate.
BAM is executed only in the following two cases:
• Serial boot mode has been selected by FAB pin
• Hardware has not found a valid Boot-ID in any Flash boot locations
If one of these conditions is true, the device fetches code at location 0xFFFF_C000 and BAM application
starts.
6.5.4.2 BAM software flow
With reference to Figure 6-4 a description of BAM logic flow is done.
PXD20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor
6-5
Preliminary—Subject to Change Without Notice