[time-nuts] T.I. questions
Hal Murray
hmurray at megapathdsl.net
Fri Feb 6 20:42:36 UTC 2015
magnus at rubidium.dyndns.org said:
> The typical noise generator chips uses a PRNG based on DFFs and XOR
> gate(s). A typical weakness is that the chain of DFFs is to short, causing
> a relatively high rate of cycling, which hearable as a beating. However, for
> some uses, that is OK.
The buzzword for the typical PRNG is LFSR - Linear Feedback Shift Register.
Many years ago, Xilinx published a good app-note on this topic. There is
also a section in Art of Electronics.
With the right generating polynomial, you get a sequence of bits that doesn't
repeat until 2^N-1 bits. The math geeks like to collect them. Each 1 bit in
the polynomial turns into an XOR gate, so you will also find collections of
polynomials with fewest bits.
It's hard to imagine serious problems with too-short. Each FF doubles the no-repeat length.
--
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