4 ms·

I'm an embedded systems programmer. We use LFSRs like everyone else. But I always make a point to feed true sources of entropy into the RNG. Stuff like this

I'm an embedded systems programmer. We use LFSRs like everyone else. But I always make a point to feed true sources of entropy into the RNG. Stuff like this if (any input changed) RNG_seed ^= sysclk; It's helped on numerous occasions where two or more identical systems co-exist (share a bus for example). Without entropy the systems RNGs are always in sync, which causes all sorts of problems. For example, on a CAN network bus collision you're supposed to back off a random amount of time. If the colliding systems RNGs are in sync, they keep colliding over and over endlessly. Entropy will kick you in the ass if you don't think about it. Which is why I always make sure to include it. You can always turn this off in DEBUG builds.

hunter2_ 3y ago
That makes sense. I could be wrong but I just have this sneaking suspicion that some systems literally have no inputs, though. Nothing received from the outside world... no HCI or sensors of any kind. But maybe such a simple system isn't likely to need much randomness? In your CAN bus example, pretty much anything transmitting onto the bus is doing so because it knows something interesting, as opposed to expressing something deterministic from its own little stored routine?
- kragen 3y ago
  not necessarily, it might be transmitting a heartbeat so you can detect that that part of the wiring harness is still turned on. i've seen embedded systems with literally no inputs, like the minipov2, but never one that transmits on a network
  - hunter2_ 3y ago
    So if you wanted a bunch of those to transmit onto the same bus/network, and use exponential backoff to resolve collisions, they would just keep colliding forever, right? Because even if they tried to seed their delay with randomness, they'd all do so the same way every time!
    - kragen 3y ago
      i think so, if they didn't have any other sources of randomness; even the traffic on the can bus would give them all the same 'random' bits if they started at the same time

hunter2_ 3y ago

That makes sense. I could be wrong but I just have this sneaking suspicion that some systems literally have no inputs, though. Nothing received from the outside world... no HCI or sensors of any kind. But maybe such a simple system isn't likely to need much randomness? In your CAN bus example, pretty much anything transmitting onto the bus is doing so because it knows something interesting, as opposed to expressing something deterministic from its own little stored routine?

kragen 3y ago
not necessarily, it might be transmitting a heartbeat so you can detect that that part of the wiring harness is still turned on. i've seen embedded systems with literally no inputs, like the minipov2, but never one that transmits on a network
- hunter2_ 3y ago
  So if you wanted a bunch of those to transmit onto the same bus/network, and use exponential backoff to resolve collisions, they would just keep colliding forever, right? Because even if they tried to seed their delay with randomness, they'd all do so the same way every time!
  - kragen 3y ago
    i think so, if they didn't have any other sources of randomness; even the traffic on the can bus would give them all the same 'random' bits if they started at the same time