4 ms·

There are things called fuses on AVRs that cannot be changed by running code but can be set and unset multiple times by an external programmer. These are appare

There are things called fuses on AVRs that cannot be changed by running code but can be set and unset multiple times by an external programmer. These are apparently different. https://en.wikipedia.org/wiki/Efuse https://en.wikipedia.org/wiki/Efuse describes the mechanism of action: "eFuses can be made out of silicon or metal traces. In both cases, they work (blow) by electromigration, the phenomenon that electric flow causes the conductor material to move."

greggsy 5y ago
I think Samsung used these for Knox?
- lucb1e 5y ago
  Correct. It's a pain in the bum. I might add that Fairphone has an official procedure to flash the original rom and re-lock the bootloader, I tried it with the FP3 at least and that worked on the first try.
- Rebelgecko 5y ago
  They're pretty common in TPM-like things
4gotunameagain 5y ago
AVR fuses are regular non volatile memory, just specifically for configuration purposes
retSava 5y ago
Aha, I was under the impression that it was simply and literally a question of passing too much current through a conducting trace internally, causing the internal resistance to overheat it, thus melting it. Perhaps that would be a method too unreliable or something. Perhaps I should read the wikipedia entry before speculating :).
- londons_explore 5y ago
  There are a bunch of ways to make efuses. Modern ones are typically flash memory that simply doesn't have the circuitry for erasure.
  - MertsA 5y ago
    That seems odd, can't high heat cause enough of an "annealing" effect to allow the charge to leak out and reset the cell? I don't know how much heating you could safely manage on a single point on the die but this would seem to be within the realm of feasible if you could uncover the portion of the die without completely destroying the packaging. If it's a plastic encapsulated IC and something like QFP where you can get at the die surface from the top then a laser could probably be used to etch through the plastic and locally heat the memory cell without trashing the chip.
    - segfaultbuserr 5y ago
      Indeed, you can. This is commonly used for microcontroller reverse-engineering and cracking - you can remove the "program read-protection" bit in some microcontrollers by exposing the fuse portion of the decapped die under UV light. That being said, in all instances of fuse manipulations I'm aware of, the chips are always decapped. I never heard of a non-destructive technique (asides from fault injection), so I'm not sure how practical is your proposal - though I won't be surprised if it exists. But make sure the chip doesn't depend on a critical bit to boot (e.g. external clock enable) before you try - indiscriminately erasing all the fuse bits can brick chips - you don't want that to happen. Also, as a matter of fact, corrupted fuse bits are certainly responsible for some hardware failures in the field. If a fuse bit ever "gets loose", it can brick many chips since their boot configurations are no longer correct. Fuses are usually designed with a negligible failure rate in normal use, still, defective chips are occasionally made. Also, if you're going to use it in a high-temperature industry application or a radiative space environment, you definitely need to be careful.
      - MertsA 5y ago

greggsy 5y ago

I think Samsung used these for Knox?

lucb1e 5y ago
Correct. It's a pain in the bum. I might add that Fairphone has an official procedure to flash the original rom and re-lock the bootloader, I tried it with the FP3 at least and that worked on the first try.
Rebelgecko 5y ago
They're pretty common in TPM-like things

4gotunameagain 5y ago

AVR fuses are regular non volatile memory, just specifically for configuration purposes

retSava 5y ago

Aha, I was under the impression that it was simply and literally a question of passing too much current through a conducting trace internally, causing the internal resistance to overheat it, thus melting it. Perhaps that would be a method too unreliable or something. Perhaps I should read the wikipedia entry before speculating :).

londons_explore 5y ago
There are a bunch of ways to make efuses. Modern ones are typically flash memory that simply doesn't have the circuitry for erasure.
- MertsA 5y ago
  That seems odd, can't high heat cause enough of an "annealing" effect to allow the charge to leak out and reset the cell? I don't know how much heating you could safely manage on a single point on the die but this would seem to be within the realm of feasible if you could uncover the portion of the die without completely destroying the packaging. If it's a plastic encapsulated IC and something like QFP where you can get at the die surface from the top then a laser could probably be used to etch through the plastic and locally heat the memory cell without trashing the chip.
  - segfaultbuserr 5y ago
    Indeed, you can. This is commonly used for microcontroller reverse-engineering and cracking - you can remove the "program read-protection" bit in some microcontrollers by exposing the fuse portion of the decapped die under UV light. That being said, in all instances of fuse manipulations I'm aware of, the chips are always decapped. I never heard of a non-destructive technique (asides from fault injection), so I'm not sure how practical is your proposal - though I won't be surprised if it exists. But make sure the chip doesn't depend on a critical bit to boot (e.g. external clock enable) before you try - indiscriminately erasing all the fuse bits can brick chips - you don't want that to happen. Also, as a matter of fact, corrupted fuse bits are certainly responsible for some hardware failures in the field. If a fuse bit ever "gets loose", it can brick many chips since their boot configurations are no longer correct. Fuses are usually designed with a negligible failure rate in normal use, still, defective chips are occasionally made. Also, if you're going to use it in a high-temperature industry application or a radiative space environment, you definitely need to be careful.
    - MertsA 5y ago