"Un-initialized" does not mean random.  It means the value is not
defined by the spec.  The value may be consistent for a production lot
of chips though.  Racing your own RC osc against the watchdog's RC
should produce a good random number.

Sherpa Doug

> -----Original Message-----
> From: Tony Pan [mailto:weidong.pan@VERIZON.NET]
> Sent: Tuesday, August 21, 2001 10:51 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Generating a "truly" random number on the
> application of power
>
>
> Thanks Frank.
>
> >>>> Using TMR0 is ok when using RC oscillator. Don't use
> TMR0 in crystal
> operation.
>
> Why?
>
> In addition, some advised me to let the WDT time-out for the
> first time
> after power then get the TMR0. When I think about it, since
> the TMR0 is
> un-initialized upon powerup, is it really necessary to go
> through the WDT
> time-out?
>
> Tony
>
> ----- Original Message -----
> From: "Wollenberg, Frank" <F.Wollenberg@GSP-BERLIN.DE>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Tuesday, August 21, 2001 10:21 AM
> Subject: Re: [PIC]: Generating a "truly" random number on the
> application of
> power
>
>
> > Tony Pan wrote:
> >
> > > Our products are used in motel environments to control ACs.
> > > The reason we
> > > need to stagger the startup is that if we don't, in case of
> > > power loss and
> > > resume, all the AC will start at the same time and that
> will cause a
> > > problem. As you know each board is situated in its room
> and working
> > > independently so there is no way one board knows what the
> > > other's doing.
> >
> > AC will start on power-resume AND power-on..
> >
> > > In my hardware we use RC oscillator. Now I use the initial
> > > TMR0 value upon
> > > powerup to get a random number. I also add the initial values
> > > of FSR, ADRES,
> > > PORTB and some ADC reads of the board into the number. I
> > > guess that would
> > > give me nearly "unique" number for each board. I then use the
> > > last few bits
> > > of this number to determine the startup time.
> >
> > Using TMR0 is ok when using RC oscillator. Don't use TMR0 in crystal
> > operation.
> > FSR and ADRES are uninit on powerup, but i think they won't differ,
> > especially if the PIC's are from the same silizium.
> > PORTB must be input, but because of the identical hardware,
> the values
> > should be identical.
> > Reading a voltage divider with ADC is OK. Use carbon
> resistors (20%). Try
> to
> > not meet the acquisition time, the Chold is varying from
> chip to chip.
> Also
> > you can use an external reference voltage from a voltage divider
> (carbon!).
> >
> > Compute this value ONLY on first powerup and use it as a
> initial seed
> value
> > of a random generator (hint: in C: persistent). Compute a
> random number as
> a
> > delay time after power becomes OK.
> >
> > > I haven't got the chance to test it yet. For we have
> other features to
> > > implement at the moment. If you see any potential
> problems with this
> > > approach, please let me know.
> >
> > It is possible to compute identical delay times, but the
> possibility is
> very
> > small.
> > Another approach is to use a "digital ID chip" from DALLAS (DS2401 i
> think).
> > This number IS unique !!!
> >
> > Frank
> > --------------------------------------
> > GSP Sprachtechnologie GmbH
> > Frank Wollenberg
> > HW-Entwicklung
> > Tel.:   +49 (0)30 769929-78
> > Fax:    +49 (0)30 769929-12
> > eMail:  f.wollenberg@gsp-berlin.de
> >
> >
> > --
> > GSP Sprachtechnologie GmbH
> > Teltowkanalstr.1, D-12247 Berlin
> > Tel.:  +49 (0)30 769929-0
> > Fax:   +49 (0)30 769929-12
> > eMail: Info@gsp-berlin.de
> > Web:   http://www.gsp-berlin.de
> >
> > --
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