[sdiy] Non maximal-length LFSR

Eric Brombaugh ebrombaugh1 at cox.net
Fri Mar 4 20:45:33 CET 2016


I've done this on DVB modems that require byte-wide scrambling. I 
started with the raw single-bit LFSR specification and then just did the 
boolean logic required to compute 8 bits per cycle. Instead of shifting 
one bit in the register you shift 8 which means you need to compute 8 
unique inputs at a time. More XOR operations at once.

I suspect that if your dsPIC implementation is having trouble computing 
a 2-tap LFSR then this might require more logic than you care to do.

Eric


On 03/04/2016 12:26 PM, Richie Burnett wrote:
> How is this done Dave?  Do you have a reference?
>
> -Richie,
>
> -----Original Message----- From: Dave Manley
> Sent: Friday, March 04, 2016 7:07 PM
> To: Richie Burnett ; Tom Wiltshire
> Cc: synth-diy DIY
> Subject: Re: [sdiy] Non maximal-length LFSR
>
> It hasn't been mentioned, but the LFSR implementation can be done so
> that it generates multiple bits per update, not just a single bit.  This
> is commonly done in telecom/ networking descramblers which operate on a
> multi-bit serdes output.
>
> -Dave
>
> On March 4, 2016 5:24:02 AM PST, Richie Burnett
> <rburnett at richieburnett.co.uk> wrote:
>> Yeah, but throwing away every 49th result would be throwing away 7
>> years
>> worth! ;-))  Could I live with that!?!?
>>
>> Thanks for everyone's help and suggestions about this.  I'm confident I
>> know
>> the way forward now, and also have some good pointers towards working
>> through the maths to back up my decisions, which always makes me feel
>> better.
>>
>> -Richie,
>>
>>
>>
>> -----Original Message----- From: Tom Wiltshire
>> Sent: Friday, March 04, 2016 12:17 PM
>> To: Richie Burnett
>> Cc: mskala at ansuz.sooke.bc.ca ; synth-diy DIY
>> Subject: Re: [sdiy] Non maximal-length LFSR
>>
>> 371 years!!
>>
>> On 4 Mar 2016, at 11:34, Richie Burnett <rburnett at richieburnett.co.uk>
>> wrote:
>>
>>> Sure the resulting sequence will be a smidgen shorter, but 2^49-1 is
>> quite
>>> long anyway.
>>
>>
>>
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