On: How the Nintendo Switch prevents firmware downgrades

[kmm]

If not by actual fuses, how is it implemented? There has to be some sort of physical change for it to survive without power indefinitely

[crankylinuxuser → kmm]

A read operation determines if the connection is "connected" or "disconnected".

It actually is litte bits of wire inside silicon that get over-current'ed and do burn away. And then a read operation returns "Disconnected".

There's no way to 'reconnect' a eFuse, in the way there's no way to reconnect an automotive fuse. So yes, eFuses are a form of hardware destruction.

[zaksoup → kmm]

I think the parent is being pedantic. All the literature I could find on eFuses (including the wikipedia article) either used 'blowing' to refer to permanent writing, or '"blowing"' (in scare quotes).

Whether or not that is descriptive of the physical process that's occurring, it certainly seems to be a reasonable term for anybody to use to talk about the functionality of eFuses.

[kevin_thibedeau → zaksoup]

Except that in many cases the "fuses" can be reset by wiping the firmware from the part and restoring it to a factory clean state.

[khedoros1 → kevin_thibedeau]

That sounds like a flag, not an eFuse. I'd expect something called an eFuse to be some form of one-time programmable non-volatile memory.

[pjc50 → kevin_thibedeau]

Then it's not really a fuse in the normal chip sense.

[kevin_thibedeau → pjc50]

The normal chip "fuses" aren't fuses anymore. They're just a section of EEPROM or flash with control circuitry altered to make them write-once only. Real fuse programming required generating high voltages and subjecting the die to high currents and localized thermal stress that modern ICs don't tolerate well.

[samuellavoie90 → kmm]

Probably just some 'Write Once' Memory, where they can raise bits to 1 but can't lower them back to 0. (Or the other way around, depending on the memory)

[rusk → samuellavoie90]

Which is typically implemented as a fuse no? I’m struggling to remember my undergrad EE but I think that’s a thing ...

EPROM back in the day used to work like this. You’d use an ultraviolet light to remake all the fuses when you wanted to wipe and start again.

Flash came along then and made these technologies obsolete.

[monocasa → rusk]

Or NOR flash.

[rusk → monocasa]

yes