AVR-Chat

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On Jan 8, 2011, at 1:39 PM, Chuck Hackett wrote:

> This is mostly a "mind experiment" because I don't have a current need but, is there
> a way to obtain a truly random (8-bit is good enough) number on an AVR? I'm not
> talking "random" in the statistically pure sense, just non-deterministic.
> 
> I'm only thinking of using it on startup. 
> 
> Sampling a "noisy diode" would be great but I want to do it without having to add
> external hardware or dedicate an I/O pin to the effort. 
> 
> All I can think of is something like setting up one of the analog inputs as 'free
> floating', turning the gain all the way up, and violating all the "how to avoid ADC
> noise" rules and taking a sample.
> 
> I don't think the timers would be helpful because these would tend to be
> deterministic and, by nature, run in sync with any code that would be sampling them.
> 
> 

I recently started a project to make electronic dice.  I'm using the libc (pseudo) random number routine.  I figured it would be could enough for my purposes provided I could come up with a random seed.  I set up a timer to free run and I read it on the first press of the roll button and pass it as a seed to random number routine.  A really fast counter with a human reaction time should provide a reasonably random seed.  I can't use just the timer and button push for all my random numbers as I need several per roll (two die and multiple random die face appearances as they "roll").  

A long time ago I setup a random number generator using three inverters in a loop.  This makes for a very fast oscillator, as fast as the logic family can go pretty much.  I fed this to an 8 bit counter connected to the CPU.  It was not synchronized with the processor clock and was quite a bit faster.  



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> 
> 

Clark Martin
Redwood City, CA, USA
Macintosh / Internet Consulting

"I'm a designated driver on the Information Super Highway"



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