> In the PIC, the answer would be to access the WDT oscillator,
> since it has its own RC oscillator that is not in sync with
> the PIC internal clock. But, it is NOT available, alas.
>

I think you can have an indirect access to the WDT counting speed. Reset
the WDT and enter an infinite loop that increments a register. When you
get a WDT reset, take the value (or last bit) from that register.

Does this make any sense ?

Tal

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Bob Axtell
> Sent: Saturday, May 31, 2003 2:28 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: A Q&D random number generator
>
>
> This application is similar to the Z80 free-running register;
> we used it as a reasonable seed value. But if you read the
> "random number" repeatedly for 16K times, you will clearly
> see a repeating, non-random pattern.
>
> In the PIC, the answer would be to access the WDT oscillator,
> since it has its own RC oscillator that is not in sync with
> the PIC internal clock. But, it is NOT available, alas.
>
> But my choice for a Q&D is to feed the TMR1 Input from an
> external RC oscillator running at 200Khz or slightly less
> (max speed for TMR1). Then simply grab the LS timer which is
> incrementing through as fast as it can.
>
> I like the single section of an 74HC14, with a resistor from
> input to output, with a cap on the input to ground. Costs
> just pennies, and you have 5 more drivers to use somewhere
> else. You now have a degree of randomness because this RC
> oscillator varies considerably with VCC, heat, component aging, etc.
>
> --Bob
>
> At 04:18 PM 5/31/2003 -0400, you wrote:
> >I'm writing a small application which needs a quick and dirty random
> >number generator.  Since the number desired will be between
> 1 and 255,
> >inclusive, it seems to me that a nice way to do it would be just to
> >start one of the timers going (I'm using the 18F452) and
> read the 8-bit
> >counter at the time I need the random number.  Since it will be
> >continually incrementing and overflowing, this would seem to
> be pretty
> >random.
> >
> >But it seems so easy (really just one instruction) that
> there must be a
> >catch. :-)  Anyone see any?  (I'm using Timer0 for something
> else, but
> >since the F452 has four timers, that is no problem).
> >
> >John
> >
> >--
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