MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
16-6 Freescale Semiconductor
16.3.2.1.1 Finding Reset Configuration Half Word
The BAM searches the internal flash memory for a valid reset configuration halfword (RCHW). A valid
RCHW is a 16-bit value that contains a fixed 8-bit boot identifier and some configuration bits (see
Section 4.4.3.5.1, “Reset Configuration Half Word Definition”). The RCHW is expected to be the first
halfword in one of the low address space flash blocks as shown in Table 16-4.
BOOT_BLOCK_ADDRESS is the first address from Table 16-4, where the BAM program finds a valid
RCHW.
If the BAM program does find a valid RCHW, the watchdog is enabled with the RCHW[WTE] bit, the
BAM program fetches the reset vector from the address of the BOOT_BLOCK_ADDRESS + 0x4, and
branches to the reset boot vector. A user application should have a valid instruction at the reset boot vector
address.
Figure 16-4. Reset Boot Vector
The watchdog timeout is set to 2.52
17
system clock periods if the watchdog is enabled.
16.3.2.2 External Boot Modes Flow.
The external boot mode is used to boot a user application from an external asynchronous memory that is
connected to the MCU external bus; it is controlled by CS0.
16.3.2.2.1 External Boot MMU Configuration
As shown in Table 16-5, the BAM program sets up the two MMU regions differently than in internal flash
boot mode. The internal flash logical address space is mapped to the physical addresses of the EBI.
Table 16-4. Low Address Space (LAS)
Block Memory Addresses
Block Address
0 0x0000_0000
1 0x0000_4000
2 0x0001_0000
3 0x0001_C000
4 0x0002_0000
5 0x0003_0000
0123456789101112131415
A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
A16A17A18A19A20A21A22A23A24A25A26A27A28A29A30A31
BOOT_BLOCK_ADDRESS + 0x0000_0004
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