If you want your boards to have a random dispersion due to component
variation, 20% resistors might not be the best choice.

Resistors are often offered in different precision grades, which is achieved
by selecting the closest-tolerance parts from a production run.  The ones
that make 1% or 5% go into the higher-priced bin, and the rest go into the
cheap bin.

The bunch in the cheap bin are unlikely to have a normal distribution about
the center value.  If the process is well-tuned and the ones coming off the
line have a nice distribution about the target value, then the cheapie parts
will have a hole in the distribution where the good ones were taken out.
They may all be within 20%, but NONE will be within 5%!

Getting your random variation from a part like a cap that isn't sold in
varying precision grades might give a more dependably random distribution of
times.

Maybe you could buy good resistors of varying values and mix your own random
selection by dumping them in a barrel and shaking them up.

> -----Original Message-----
> From: Wollenberg, Frank [mailto:F.Wollenberg@GSP-BERLIN.DE]
> Sent: Tuesday, August 21, 2001 9:21 AM
> To: PICLIST@MITVMA.MIT.EDU
> 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
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>
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