NXP Semiconductors PXN2020, Reference Manual, Page: 261 / 1376

Boot Assist Module (BAM)
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 9-7
Figure 9-3 shows the fields of the RCHW.
If the BAM fails to find a valid RCHW, it assumes the flash is erased or corrupt and switches to serial-boot
mode.
If the BAM finds a valid RCHW, the configuration bits are parsed as shown in Table 9-4 . The BAM then
fetches the reset vector from the address of the RCHW + 4, and program execution continues from that
address.
Offset: BOOT_BLOCK_A 0x0000 Access: User read-only
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 WTE 0 VLE BOOT ID
W
Reset 0 0 0 0 0 0 0 0 0 1 0 1 1 0 1 0
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
W
Reset 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Figure 9-3. RCHW Fields
Table 9-5. Internal Boot RCHW Field Descriptions
Field Description
WTE Watchdog timer enable. This bit determines if the software watchdog timer is disabled.
0 Disable software watchdog timer
1 Software watchdog timer maintains its default state out of reset, (i.e.,enabled).
VLE VLE Code Indicator. This bit is used to configure the MMU to execute the user code as either Classic Book E
code or as Freescale VLE code.
0 User code executes as classic Book E code.
1 User code executes as Freescale VLE code.
BOOTID Boot identifier. This field serves two functions. First, it is used to indicate which block in flash memory contains
the boot program. Second, it identifies whether the flash memory is programmed or invalid. The value of a valid
boot identifier is 0x5A (0b01011010). The BAM program checks the first halfword of each flash memory block
starting at block 0 until a valid boot identifier is found. If all blocks in the low- address space of the internal flash
are checked and no valid boot identifier is found, the internal flash is assumed to be invalid and a CAN/SCI boot
is initiated.