Random outcome on multiple devices

[Aussie Barry]

Hi All,

I have read most of the forum posts regarding Random Number generation but can't find one that answers my particular question.
I understand the random number sequence will be highly repeatable and predictable if using the same seed but what is the likelihood of the same random number being generated on two or more stand alone PIC's?
For example, four PIC12F508's all running the same code and all turn on at the same time - would the outcome of a "RANDOM MyNumber" command likely to be the same for all four PIC's?

A bit of background - I need to build an array of 32 pairs of LED's and set each pair to toggle at a different frequency. Each pair of LED's will be driven by its own PIC. I was hoping to be able to introduce a degree of randomness to the whole scheme rather than having to program each PIC differently to achieve an overall variation in flash rates between individual pairs.

Comments and suggestions would be greatly appreciated.

Cheers
Barry
VK2XBP

[Art]

Hi Barry, Greetings from Australia

If you run the same program on all four pics the result will be the same.
There is no math formula to produce a random number
PicBasic will cycle through a finite set of numbers.

If you can choose a seed, you could load each pic with a different seed.

If any action depends on a user pressing a button you can time the button
push, or count contact bounce, and add that result to a seed for a different seed each launch.

Without any external input, I think you'd be needing to at least write
something different to each on-board EEPROM,
unless you can do something really tricky to detect noise in a power supply or some such thing.
Cheers, Art.

[Aussie Barry]

Hi Art,

In what part of the "Land Down-Under" do you reside?
I am in Sydney's southern suburbs.

Thanks for the quick reply.
Noise detection systems are overkill for this project. I was hoping that the power up sequence for individual devices would have provided some degree of variation.
What if I used a timer based system to generate the seed and then de-tuned the internal oscillator between various devices to get some spread of variation - do you think that would work?
Alternately, I could cascade a pulse from PIC to PIC and emulate a button push... just thinking out aloud here.

Cheers
Barry
VK2XBP

[Art]

Close to Brisbane here

There are always things you can try. Maybe PicBASIC's POT command.
only instead of using a potentiometer, try a fixed value resistor,
and maybe a reading with POT will vary enough with temperature
(if you had to produce multiple units and keep firmware identical).

Once with the Sony PSP I got random numbers by looking at the audio input

[HenrikOlsson]

Hi,
It all depends on where you want to introduce the variation.... If you by "detune" the oscillator mean to tweak the OSCTUNE register slightly between devices then you might as well program in a different seed from the get-go - there will be a unique program in each device either way.

Perhaps you can use something like the RCTime command with an external RC (duh...) network to generate the seed and rely on the tolerance of the R and the C to get the needed variance.

/Henrik.

EDIT: Art beat me to it while I was typing and doing some other things.

[Aussie Barry]

Great suggestions guys - thank you.

I will look at the POT/RCTime commands and choose wide tolerance components or random values to achieve the desired variation.

Cheers
Barry
VK2XBP

[Art]

I can't help thinking there would be some noise on the power you'd see with ADC.
Interference between ADC pins has always been a problem...