[sdiy] Shift register sequence period

[Neil Johnson]

Tim,

Interesting circuit.

Looking at it briefly, a couple of comments:

1/ I think the start circuit is upside down - pin 13 of the 4030  
should start HIGH (giving you an inverter) to overcome the all-zeros  
startup state.  As is shown in the schematic at the start there is no  
inverter and so the all-zeros state is maintained.  Its only when the  
0u47 cap has charged up does it get out of the all-zeros state.

2/ Looking on the 'net for 18-stage maximal length tables I don't see  
anything that corresponds to this 3-tap design.  For maximal length  
there is a single 2-tap and many 4-tap choices (some of which can be  
implemented with the 4006), but nothing for 3-tap.

Of course, I may be wrong :)

Neil

On 5 May 2008, at 20:27, Tim Stinchcombe wrote:

> Hi list,
> 	I've been trying to establish the length of the sequence from a
> particular 4006 shift register setup. It is used in the Doepfer  
> A-117, which
> is where I came across it; Ken Stone also uses it in his 'Digital  
> Noise'
> module:
>
> http://www.cgs.synth.net/modules/cgs31_digital_noise.html
>
> and the earliest reference I've found to it so far is in a 1990  
> book by Ray
> Marston, 'Integrated Circuit and Waveform Generator Handbook'.
>
> At first I thought the inverter in the feedback (middle 4030 gate  
> in Ken's
> circuit above) would mean that the length of the sequence would be  
> some what
> less than 2^18-1. Then in attempting to translate the gates into  
> polynomials
> (if I've got it right) the inverting action seems to make the feedback
> non-linear, and so the normal theory of LFSRs no longer applies. So I
> 'captured' a few bit sequences on my scope (the longest being 59  
> bits), and
> ran it through the Berlekamp-Massey algorithm, which appears to say  
> that the
> length of the minimum *linear* feedback shift register which could  
> generate
> this sequence is actually 19 stages, i.e. one more than the 18  
> available in
> the 4006! I'm hoping that this 'anomaly' *is* due to the non- 
> linearity...
>
> Has anyone done any analysis of this or similar circuits, i.e. with an
> inverter in the feedback? Or if anyone steer me in the right  
> direction, or
> toward a decent analysis of this circuit, then I'd appreciate it.  
> (I can't
> find anything similar in Electronotes; not much seems to turn up in  
> the
> Synth DIY archives; but I note the EDP Wasp and Gnat use similar  
> circuits,
> but with a different arrangement of taps and inverter.)
>
> Cheers,
> 	Tim

--
http://www.njohnson.co.uk