Fallback and MultiBoot Flow - 2020.2 English

In the Zynq® UltraScale+™ MPSoC, the CSU bootROM supports MultiBoot and fallback boot image search where the configuration security unit CSU ROM or bootROM searches through the boot device looking for a valid image to load. The sequence is as follows:

• BootROM searches for a valid image identification string (XLNX as image ID) at offsets of 32 KB in the flash.
• After finding a valid identification value, validates the checksum for the header.
• If the checksum is valid, the bootROM loads the image. This allows for more than one image in the flash.

In MultiBoot:

• CSU ROM or FSBL or the user application must initiate the boot image search to choose a different image from which to boot.
• To initiate this image search, CSU ROM or FSBL updates the MultiBoot offset to point to the intended boot image, and generates a soft reset by writing into the CRL_APB register.

The following figure shows an example of the fallback using the MultiBoot flow.

In the example fallback boot flow figure, the following sequence occurs:

• Initially, the CSU bootROM loads the boot image found at 0x000_0000.
• If this image is found to be corrupted or the decryption and authentication fails, CSU bootROM increments the MultiBoot offset by one and searches for a valid boot image at 0x000_8000 (32 KB offset).
• If the CSU bootROM does not find the valid identification value, it again increments the MultiBoot offset by 1, and searches for a valid boot image at the next 32 KB aligned address.
• The CSU bootROM repeats this until a valid boot image is found or the image search limit is reached. In this example flow, the next image is shown at 0x002_0000 corresponding to a MultiBoot offset value of four.
• In the example MultiBoot flow, to load the second image that is at the address 0x002_0000, MutiBoot offset is updated to four by FSBL/CSU-ROM. When the MultiBoot offset is updated, soft reset the system.

The following table shows the MultiBoot image search range for different booting devices.