PXS20RM Datasheet, PDF (148/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Boot Assist Module (BAM)
The communication is done in half duplex manner, any transmission from host is followed by the MCU
transmission:
• Host sends data to MCU and starts waiting
• MCU echoes to host the data received
• Host verifies if echoes are correct
— if data is correct, the host can continue to send data
— if data is not correct, the host stops to transmit and MCU need to be reset.
All multi-byte data structures are sent with MSB first.
A more detailed description of these steps follows.
8.4.2.5 Download 64-bit password and password check
The first 64 bits received represent the password. This password is sent to the Password Check procedure
which verifies if it is correct.
Password check data flow is shown in Figure 8-3 where:
• SSCM_STATUS.PUB = 1 specifies serial boot mode with public password
• SSCM_STATUS.SEC = 1 specifies that the flash memory is secured
In case of serial boot mode with public password, the received password is compared with the public
password 0xFEED_FACE_CAFE_BEEF.
If public access is not allowed but the flash is not secured, the received password is compared with the 64
bit value saved in the NVPWD0 and NVPWD1 locations in the shadow flash.
In both cases, comparison is done by the BAM code. If it fails, BAM pushes the device into static mode.
If the public password is not allowed and the flash memory is secured, the received password is compared
by hardware against the password stored in NVPWD0 and NVPWD1. Only in this case (no public
password and secured flash), the words of the provided password must be swapped
(NVPWD1|NVPWD0).
After a fixed time waiting:
• If the correct password was supplied, flash is now unsecured and BAM continues its task
• If an incorrect password was supplied, flash is still secured; BAM puts the device into static mode
PXS20 Microcontroller Reference Manual, Rev. 1
8-6
Freescale Semiconductor