On Wed, 14 May 1997, John Payson wrote:

> > At 17:17 13/05/97 +0000, you wrote:
> > >I need a routine that returns a random number to implement something
 similar
> > >to CSDMA/CD. I need it to make a random wait time after a collision, i've
> > >seem some routines but how do i get different seeds to inicialize the
> > >routine ?
> >
> >  Hello,
> >  I've found that performing a XOR operation with all the un-inisialized
> > values of the user RAM at startup (in the PIC16C84), you get a good random
> > seed.
>
> Yes, but if two PICs happen to XOR to the same value you're sunk.  Even
> using a 2-byte XOR of user RAM is no guarantee of safety; depending upon
> the characteristics of the chips you may find that certain chips tend
> toward the same stuff.  For a CSDMA/CD application, that would be a
> killer.
>

I think i found a solution to make a random seed generator, at least one
to solve this problem. It's something like this:

       Vdd
        |
        -         --------         The function will work like this:
       | |       |   PIC
       | |       |                    1.  Pin R (output) = 0
        -        |                    2.  Pin R (input)
        |        |                    3.  Start counter
        +--------| R                  4.  While R == 0 increment counter
        |        |                    5.  Stop counter
      -----      |                    6.  Return counter
      -----      |
        |                  We only need to use some very cheap components
      -----                (with big tolerances) to get a fair random
                           seed generator :-).

Antonio Almeida