Re: How to develop a random number generation device

On Sat, 08 Sep 2007 23:14:52 GMT, Rich Grise <rich@xxxxxxxxxxx> wrote:

On Fri, 07 Sep 2007 21:07:28 -0700, John Larkin wrote:
On Fri, 07 Sep 2007 17:33:06 -0700, "slebetman@xxxxxxxxx"

What's wrong with using the regular XOR feedback but simply invert the
output? Surely that eliminates the possibility of lockups (allowing
all zero state) and uses just one extra NOT gate:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
.--|>o--|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|
| | | | |
'------------------------X---X-----X---'


Then all 1's is the lockup state.


The Xilinx app note says that using the all 1's lockup state is
recommended, (i.e., XNOR) because the flip-flops' default powerup
state is 0, which is in the sequence, so it doesn't lock up by
default. ;-)

Cheers!
Rich

Shift registers are very cheap in Xilinx chips (you can use the cell
config bits) so you can make huge sequences. And you can initialize
any shift register to any pattern, so an xor or xnor is equally safe,
as long as you init it to something random-ish.

We just did an FPGA that contains eight channels of Gaussian noise
generator. Each channel uses, as I recall, eight *long* shift
registers of different sequence lengths, with a 16 bit random number
manufactured by scrambling bits from various registers. Every shift
register is initialized to something different. Clocked at 128 MHz, no
channel will repeat a pattern in the life of the universe.

The 16-bit random words are digitally lowpass filtered to produce a
nearly-Gaussian probability distrib and programmable -3 dB point, from
mHz to 2 MHz.

John



.

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