At 09:54 PM 10/11/2004, Martin McCormick wrote:

>         I have heard of shift registers being used along with feedback
>to generate a pseudo-random bit pattern.  A PIC would be excellent for
>setting up such a scheme because any two or three bytes can be shifted
>left or right and ganged in to a shift register so that a bit at one
>end marches through the register and ends up at the other end.  How
>does the feedback loop work to scramble the bits so that the output at
>least appears random?

Someone mentioned my AVR code that does this, easily adaptable to the PIC.

>         If the bits in the shift register start out as all high or all
>low, will that prevent the entropy from starting?  In the project I am
>thinking of, even the apparence of randomness is good enough.

The only thing you can't do with a maximal length generator, is to start it 
with all zeroes. That's a "stuck" state.  Anything else you load in will be 
fine.
That means that an N bit register can only generate (2^N)-1 conditions, but 
you can always make the PN gen longer, and then take some bits out of the 
middle, and those will generate all 256 possible states with equal 
distribution. 

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