Re:How does USB device discovery work? [video]
by amelius on Sunday July 11, 2021 @05:31PM
(#27804653)
> It's probably more of a bandwidth issue than a memory issue.
Yeah, but you can use multiple banks and write in a round-robin fashion.
> Depending on the signal characteristics this can actually be worse.
Of course, that was only an example. You should probably do something like runlength encoding.
In any case, the USB protocol is much slower than modern hardware can handle, so certainly doable.
Re:How does USB device discovery work? [video]
by AussieWog93 on Sunday July 11, 2021 @07:20PM
(#27805340)
Consumer grade DDR4 modules have a bandwidth of 25.6 GB/s. Even at 10 samples per bit you're laughing.
It's probably more of a bandwidth issue than a memory issue. USB 2.0 runs at 480 Mbit/s, so if the oscilloscope can decode that, it would have to be able to output/store at least that much in terms of bandwidth. A USB keyboard would never hit that type of bandwidth, but presumably a buyer of such expensive oscilloscope would expect the feature to work every time, rather than only in best case scenarios.
>And note that (since it's a digital signal) you don't have to sample evenly; you can also record the timepoints of the edges.
Depending on the signal characteristics this can actually be worse. eg. if it switches very rapidly you'd probably be better off storing the reading as PCM or something, because the timestamps (which are presumably nanosecond level) would take more space to record than the raw sample reading (a few bits at most).