Re: 8 Bit Random Numbers

On Sun, 01 Feb 2009 22:36:15 +0000, Nobody <nobody@xxxxxxxxxxx> wrote:

On Sun, 01 Feb 2009 10:17:46 -0600, John Fields wrote:

What you cannot do is make an XOR-based
PRNG that pseudorandomly cycles through all values from 0 to 255.

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Not true.

If you run the LTspice circuit list or the BASIC program I posted
earlier (or build it, even) you can see all 256 states exercised.

Your LTspice circuit isn't an "XOR-based PRNG" (i.e. LFSR), as it includes
2 (inclusive-) OR gates.

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Sorry, but that's not right.

It's EXOR _based_ because the basic polynomial is generated by A4, A10,
and A11 (EXOR gates) with taps along the register located at Q3,Q5,Q6,
and Q8.

Furthermore, A12 and A13 don't comprise 2 inclusive-OR gates, they
resolve to a single inclusive NOR
---

If you're limited to XOR gates, the all-zeros state is bound to be
invariant. That's the "linear" part of LFSR.

---
Duh.

The point was that with the addition of the extra circuitry, (A12, A13,
and A14) slipping into the lockup state by accident becomes impossible,
even if, on purpose, the counter is cleared on power-up.

JF
.

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