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k210-sdk-stuff/doc/otp_layout.md
Wladimir J. van der Laan b641ee4054 doc: OTP fuses A/B
2019-05-07 19:35:16 +02:00

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OTP layout

Some random notes about the layout of the Kendryte K210 OTP memory.

There is some on-board One-Time Programmable Memory, which is used to configure the boot sequence as used by the boot ROM. A part of it is pre-written in the factory.

The total size seems to be 16384 (0x4000) bytes.

Ofs   Type   Description
----------------------------------------------------
0000  u32    "ROXM" (0x4d584f52) magic value
0004  u16    Size of "boot patch" program
0006  u8[sz] Program data to be copied
3d91  u8[11] Looks like a production run identifier (ASCII)
3d9c  u32?   Serial number? Differs per board instance
3da0  u16    ?
3da2  u16    ?
3da4  u16    ?
3da6  u16    ?
3db0  u8[32] Expected SHA256 hash of firmware
3fd0  u8[16] 2 bits per entry, for 64 entries
3fe0  u8[16] 2 bits per entry, for 64 entries
3ff0  u8[16] Causes read errors. Might be write-only area for encryption key.

  • The "boot patch" program is copied to SRAM at address 0x805f8000 at boot and executed. I think it's meant to be a vector to fix issues in the ROM without having to modify the chip.

  • You can read the OTP of your board using the otp_dump tool in this repository. Note that the OTP contains a serial number (at least 0x3d9c..0x3d9f seem to differ between boards) so it'd be wise to treat the output as privacy-sensitive.

  • My intuition is that the areas at 0x3fd0 and 0x3fe0 are bit fields that specify which parts of the OTP have been written and which have not.

  • Be careful: any writing to the OTP (if possible at all, I don't know, there are some routines in the ROM that look like they might do this but haven't tried !) might brick your chip unrecoverably.

Fuses A

OTP register 0x68 has some bits that affect the boot process. These might be one-time programmable fuses.

The following bits are known:

Bit      Description
-------- ------------------------------------------------------------
1        If set, allow entering ISP even if Fuses B bit 7 disallows it, force normal boot

Fuses B

OTP registers 0xbc (=bits 32..63) and 0xb8 (=bits 0..31) affect the boot process. These might be one-time programmable fuses.

Bit      Description
-------- ------------------------------------------------------------
0        checked in unreached ROM code, specific use unknown
7        disallow entering ISP
8        skip decryption of firmware
9        check SHA256 against OTP 0x3db0
61       if bit 62 not set: if set, boot only SPI0 (external), else only SPI3 (internal)
62       try booting SPI0 (external) flash first then SPI3 (internal) flash
63       determines kind of boot (sets higher frequencies-might be the TURBO MODE mentioned in the datasheet)