Power applied to the SoC
When chip is powered on the dram controller pushes the address of brom (boot rom, mask rom that is hardwired) to PC (program counter) and execution of rom begins.
- UART1 gets initialized to some predetermined baudrate. Baud usually depends in chip input clock source. Observation has been made that UART1 is disabled on many phones, but can be enabled if proper firmware is used and eFuses are not blown.
- Initialize storage with low clock. Reason why it's using low clock is because muxes are not programmed at this stage.
- Wait for "Start" string on UART1 for unknown amount of time, the internet keeps buzzing about 150ms. After start is recieved, listen to commands from host until jump is observed (refer to brom spec in further reading section). If start is not sent then Preloader image will be loaded from the storage to the SRAM (static ram) and brom loads it according to the flags set in header.
- If KCOL0 pin is shorted to the ground, preloader loading is avoided and download protocol is rolled up on USB port.
This format is used in brom and preloader(has to be verified). All fields are in little endian.
File info image format
|4||Length of complete header (including this size field and the magic field above)|
||Pay attention to padding bytes|
|2||Image type||0: NONE 1: ARM-Bootloader 2: ARM-External-Bootloader 10: Root-Certificate 256: Primary-MAUI 264: VIVA 769: SECURE_RO_ME|
|1||Storage type||0: NONE 1: NOR Flash 2: NAND Sequential Flash 3: NAND_TTBL 4: NAND_FDM50 5: EMMC-Boot-Region 6: EMMC-Data-Region 7: Serial Flash 255: Device-End|
|1||Signature type||0: No Signature 1: PHASH 2: SINGLE 3: SINGLE and PHASH 4: MULTI 5: TYPE_NUM 255: TYPE_END|
|4||Total file size|
||Maximum file size|
|4||Content offset in file||You can have multiple headers before actual content (preloader assembly image)|
|4||Signature length||If no signature was set, size is 0|
|4||Jump offset||Seems to be the same as content offset, maybe because preloader needs to execute|
|4||Ending||1: POST_BUILD_DONE 2: XIP (Execute In Place) use bitwise OR to combine multiple flags|
Example data from MT6735P chipset
ROM:00031128 magicValue DCD 0x14D4D4D ROM:0003112C HeaderSize DCD 0x38 ; completeHeaderSize ROM:00031130 aFile_info DCB "FILE_INFO",0,0,0 ROM:0003113C alwaysOne DCD 1 ROM:00031140 FileType DCW 1 ; ARM-Bootloader ROM:00031142 FlashType DCB 5 ; EMMC-Boot-Region ROM:00031143 SignatureType DCB 3 ; Single and PHASH ROM:00031144 LoadAddress DCD 0x200D00 ; SRAM address ROM:00031148 FileLength DCD 0x1CE4C ROM:0003114C MaximumSize DCD 0x40000 ROM:00031150 ContentOffset DCD 0x300 ROM:00031154 SigLen DCD 0x124 ROM:00031158 JumpOffset DCD 0x300 ROM:0003115C Ending DCD 1 ; POST_BUILD_DONE
- Coolpad Modena 2 (coolpad-modena2)
- https://forum.xda-developers.com/u/repair/coolpad-sky-3-e502-modena-2-unbrick-t3610163 (KCOL0 unbrick)
- Bootrom specification