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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