[Cryptech Tech] Noise board on Novena

[Benedikt Stockebrand]

Hi Bernd and list,

Bernd Paysan <bernd at net2o.de> writes:

> The disadvantage of USB is that it is way more complicated... if you implement 
> USB on an FPGA, I'd suggest you use a microcontroller core+USB combination.

right (at least as far as I can tell), but I was more thinking about
adding a USB interface to a microcontroller, not an FPGA.

>> Well, there's the issue with the speed; if you use automatic baud rate
>> detection then you have to make sure you discard that first bit.
>
> Actually, the first bit you use to measure the baud rate is the start bit.  
> The next bit has to be a 1 to get an edge at the end of the start bit, so you 
> can do that measurement.  The bit itself will be sampled correctly (you sample 
> bits in the middle of the cycle).

That's not what I meant, but I think you may miss the background:
Currently I send the raw entropy stream out of the serial interface.  If
the first bit has to be set to a fixed value, then it has to be
discarded on the host/receiving side.

Otherwise you are correct; question then however is how the speed
detection is implemented in various OSes and such.  Again, I'm not
talking FPGA here; with an FPGA it is likely the easiest to configure
the speed on both ends.

> If you use the "entropy only for initial seed" approach, shutting down the 
> diode current may be a good idea.

Yes, it may.  But it also makes the firmware or FPGA design more
complex.

And what's worse, you have to ensure somehow that the thing is up and
running again when you power it on.  Right now I do some "extensive"
testing at startup (for those who've got a board: That initial blinking,
when the LEDs are on is when things are being tested); overall time
should be around 250ms for those tests.  That's ok for initial power-up,
but not for power consumption.


Cheers,

    Benedikt

-- 
Benedikt Stockebrand,                   Stepladder IT Training+Consulting
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