Random outcome on multiple devices

[Art]

If the device is mains powered there's a 50/60Hz clock on the transformer secondary
the way old LED digital clocks used to keep time from mains signal.
If you are looking at that with a pic that has an independent clock the results will vary.

If you can run an ADC from internal RC clock

DEFINE ADC_CLOCK 3

and the pic from xtal, and look at any clock or output signal
generated by the pic program with ADC the results will differ as clocks lose sync.

I would try the resistor Cap network first (timing cap discharge via resistor).

[Demon]

(Never done this)

What about reading a resistor on analog pin? A high value, maybe 100K, would that produce a reasonably wide range of seeds?

Robert

Edit: since no 2 resistors are exactly alike, the voltage should vary.

[Art]

It sounds like it would work.
That would be the first thing I'd try now, nice one.
.. and if not, then tapping the centre of a divider (two resistors from +ve to Gnd).
Still cheaper than timing a cap discharge.

I can't say I've tried to make two devices produce a different result,
just one device from one launch to the next.

(Never done this)

What about reading a resistor on analog pin? A high value, maybe 100K, would that produce a reasonably wide range of seeds?

Robert

Edit: since no 2 resistors are exactly alike, the voltage should vary.

[Demon]

Even better, use a LED and 330R resistor. You're bound to get different voltages and it serves as a power indicator.

Robert

[Aussie Barry]

Hi Guys,

All great ideas - thank you.
I have decided to program each PIC separately making subtle differences to LED flash rates and "off-time" periods to give an overall effect of randomness.
This is a one-off project that I am doing as a labour of love for my company. As such, I just want to get in and get out as quickly as possible

Cheers
Barry
VK2XBP

[HenrikOlsson]

Hi,
One possible issue with using the ADC is that the 12F508 being mentioned in the first post doesn't have one....

/Henrik.

[Art]

Still in general, it sounds like something to try.
You get 14 bit ADC with pic if I remember,
should be high enough resolution to vary with resistor tolerance.

I was keen to see how it panned out without doing it myself

[HenrikOlsson]

As long as we stick to 8-bit PICs (which is what PBP supports) there's a quite a selection devices with 12bit ADC and two devices with 16bit ADC (18F8xJ72). All of them are enhanced 16F1xxx or 18F parts though, the smallest I could find thru a quick look is 28pin.

/Henrik.