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
>
>
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