I think the best bet would be to time the WDT period against the
crystal/resonator, as this will vary slightly from part to part and
with voltage and temperature. Using TMR0, you can measure the period
to 1uS at 4MHz, i.e. about 0.005% of the nominal WDT period - it is
reasonable to assume that WDT periods will not be matched to this
degree.

I'd do it like this  :=20

On reset :=20
Check STATUS to determine if reset was power-up or WDT:

If Power-up :=20
  Set TMR0 to internal clock, no prescale.=20
  Allow WDT to time out.

If WDT timeout :=20
  TMR0 should contain a fairly random value, at least in the lowest
few bits.

If you want more randomness, increase the WDT prescaler setting, to
measure a longer measurement period.=20
The TMR0 register may also have initialised randomly as well, which
will help.=20



On Tue, 14 Aug 2001 13:17:59 -0400, you wrote:

>Hi,
>
>In our project, several identical boards are connected together. =
Normally,
>on the application of power, they will be turned on at the same time and
>starting running at the same time. But we don't want that to happen. If =
all
>boards are running at the same time, they will consume too much power =
and
>cause a problem.
>
>So, we want to impose a lockout time at the beginning of the software; =
the
>lockout time should be random for each board (though they use the same
>software). Only after the lockout time expires would the program start
>running. Thus the boards will start at different time and won't cause a
>power shortage.
>
>My question is: how to generate a "truly" random number on the =
application
>of power. The number should not be determined upon the compilation of =
the
>software, but should be upon the initial powerup of the chip. (Or, it =
should
>be determine by each individual chip.)
>
>Help?
>
>(By the way, I write my program in C and use PICC compiler.)
>
>Thanks in advance.
>
>Tony Pan

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