SRK OTP Fuse Value Changes from 0 to 1 When Rewriting After Initial Programming

[Karun]

Hello NXP Support,

I am currently working on programming the SRK OTP fuses on i.MX6ULL.

Initially I write fuses using UUU tool by adding below commands in .auto script.

FB: ucmd fuse prog -y 3 0 0xXXXXXXXX

During testing, I tried rewriting an already programmed OTP fuse, using the fuse prog command from the U-Boot shell.
I observed that after attempting to rewrite, the fuse value changed slightly — for example, from 0x057a6cf3 to 0x057a6cf7 to 0x057a6cff.

From my initial analysis, it seems that the OTP fuse allows changing bits from 0 to 1, but not from 1 to 0, which matches typical OTP behavior (one-time programmable: only setting additional bits, not clearing).

Question:

1. When we are dealing with SRK hash, it should not change once written into OTP. but It is changing, so how to prevent it?

Please let me know if further details are needed.

Thank you for your support

[Harvey021 · Accepted solution]

Hi @Karun 

We can program the SRK_LOCK. Please have a reference - here 

Regards

Harvey