[sdiy] Pseudo Random Noise Generator in an AVR?

[Naoki Iwakami]

Hi Jim,

I did it using Arduino before.  I used Linear Feedbak Shift Register.
http://en.wikipedia.org/wiki/Linear_feedback_shift_register

The source is available in Ongaku.h in following zip file, but it
might not be very useful since the source requires a shield I
developed.
http://gaje.jp/resources/ongakushield/OngakuShield_20100514.zip

In short, the algorithm is like following,

volatile uint16_t lfsr = 37892;

run_this_periodically() {
            uint8_t m = (lfsr >> 12) & 0x1;
            uint8_t n = (lfsr >> 15) & 0x1;

            lfsr <<= 1;
            lfsr |= m ^ n;

            output_to_port(n);  // n is 0 or 1.  Set some output port
when n==1 and reset if n==0
}

I used timer overflow interruption to invoke the update cycle.
The noise quality would be good with high update rate, but I don't
remember the number, sorry (maybe around 10kHz?)
Lowering the clock is also interesting.  That gives flanger-like effect.
ATTiny is good enough since the algorithm is very simple.

Cheers,
-- Naoki

On Tue, Jan 8, 2013 at 9:10 PM, Jim Patchell <patchell at cox.net> wrote:
> I am trying to elimnate parts in my current project.  I have every thing on
> the board, except the analog noise generator (you know, reversed biased e-b
> junction.  Gaining enough realestate may not be possible, and I don't want
> to add any more square inches.  So, I was thinking, why not move the noise
> generator into the ATmega that is also on the board.  Making a 32 bit linear
> feedback shift register in the firmware should not be a problem.  Running it
> fast enough might be  I would want to run it at a minimum of 40K shifts per
> second.  Not even sure if that is fast enough.
>
> Anybody else every try something like this, and how did it work. Otherwise,
> I am going to have to break out the old STK500/501 and write some code. ;^)
>
> -Jim
>
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-- 
岩上直樹
Naoki Iwakami