User Application
Image linked to
address 0x0100_0800
Image SHA pattern 20 Bytes
Auto appended by
ImageCreator tool
Application code
Initial Stack Pointer (SP)
Reset Vector (PC)
Programming, Bootstrapping, and Updating the Flash User Application
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SWRU543–January 2019
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On-Chip Parallel Flash
21.8 Programming, Bootstrapping, and Updating the Flash User Application
Direct programming of the on-chip flash using JTAG or flash-loaders is not supported by the CC3235SF
device. During programming, the image is always first downloaded onto the serial flash, using the filename
/sys/mcuflashimg.bin. During the first-time boot, the bootloader reads the encrypted image from serial
flash, decrypts it, and writes it onto the on-chip flash. The bootloader ensures that the on-chip flash image
is always in sync with that on the serial flash. Otherwise, if a valid image is detected on the on-chip flash,
the bootloader will pass on the execution control to that image.
The user application image binary should be linked to run from 0x0100_0800.
Figure 21-12 shows the final structure of the image on the serial flash, where the initial 20-byte SHA-1
hash is auto-computed by the image creator and appended to the image provided by the user.
Transferring the image from the serial flash to the on-chip flash is a multistep process. The process starts
by detecting the /sys/mcuflashimg.bin file on the serial flash. When the file is detected, the whole image is
transferred to on-chip flash, skipping the first 20 bytes. A SHA-1 hash for the transferred portion is
computed and verified against the SHA-1 at offset 0 on the serial flash file. Finally, the header is written
onto the on-chip flash offset 0, to identify the image as valid.
The first 20-byte hash part of the serial flash file /sys/mcuflashimg.bin is not copied nor does it take part in
the generation of SHA-1 by the bootloader. The first 20-byte hash serves as an identifier for detecting a
new image on serial flash. A mismatch with the previously stored hash triggers an update cycle, and the
transfer process is repeated.
Figure 21-12. User Application Image Binary Structure on Serial Flash
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