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doubts on LFSR(linear feedback shift register) used in BIST

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neetinsingh

neetinsingh

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i have following doubt related to LFRS

1. How we decide for the TAP point in a CUT(circuit under test)?
2. How we choose cherterstic polynomial for lfsr?

Also tell me how we decide the width of Signature register in BIST?
 

Re: doubts on LFSR(linear feedback shift register) used in B

Hi Neetin,


2. How we choose cherterstic polynomial for lfsr?

The characteristic plynomial is the LFSR connection which will give the maximum number of random values before it starts to repeat the values. Each LFSR will have a characteristic polynomial, which is pre-determined. There are equation to find that, which I don't remember just now.


Some Polynomials for Maximal LFSRs
https://en.wikipedia.org/wiki/Linear_feedback_shift_register

You can see the polynomials for the different length of the LFSRs is given in the link above.

1. How we decide for the TAP point in a CUT(circuit under test)?
I didn't understand the question... :-(

-cheers
vlsi_eda_guy
 

Re: doubts on LFSR(linear feedback shift register) used in B

vlsi_eda_guy said:
Hi Neetin,


2. How we choose cherterstic polynomial for lfsr?

The characteristic plynomial is the LFSR connection which will give the maximum number of random values before it starts to repeat the values. Each LFSR will have a characteristic polynomial, which is pre-determined. There are equation to find that, which I don't remember just now.


Some Polynomials for Maximal LFSRs
https://en.wikipedia.org/wiki/Linear_feedback_shift_register

You can see the polynomials for the different length of the LFSRs is given in the link above.

1. How we decide for the TAP point in a CUT(circuit under test)?
I didn't understand the question... :-(

-cheers
vlsi_eda_guy
Click to expand...

hi

TAP= Test access point......

it is the point from where u take a feed back from the output of a latch in LFSR chain. this point is being stated using a polynomial equation which i didnt get.

if u want i can tell u abt the book from which i had refered it..........
 

Re: doubts on LFSR(linear feedback shift register) used in B

First thing, one clarification for the last mail, the characteristic polynomial equation which gives the maximum sequence of bits before repeat is called "Primitive Polynomial".

Ok, so the TAP points are not from the CUT, it is from the LFSR. These TAP points are decided by the characteristic polynomial equation.

From the polynomial equation of the LFSR you can see where the TAPs has to be. What ever power of X is there in the equation the output of that flop will be tapped and then send back to the first flop.

I hope you know there are two types of LFSR internal/external.

You can refer to the following for more details

**broken link removed**

Try to understand the figures in the explanation ...

-cheers
vlsi_eda_guy
 

Re: doubts on LFSR(linear feedback shift register) used in B

vlsi_eda_guy said:
First thing, one clarification for the last mail, the characteristic polynomial equation which gives the maximum sequence of bits before repeat is called "Primitive Polynomial".

Ok, so the TAP points are not from the CUT, it is from the LFSR. These TAP points are decided by the characteristic polynomial equation.

From the polynomial equation of the LFSR you can see where the TAPs has to be. What ever power of X is there in the equation the output of that flop will be tapped and then send back to the first flop.

I hope you know there are two types of LFSR internal/external.

You can refer to the following for more details

**broken link removed**

Try to understand the figures in the explanation ...

-cheers
vlsi_eda_guy
Click to expand...

hi

link u had given is good. but my doubt is still not clear.

using following img i would like to explain wht exactly my doubt is..



thnkx
 

Re: doubts on LFSR(linear feedback shift register) used in B

use this document, it may be helpfull to u.
 

    neetinsingh

    neetinsingh

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Re: doubts on LFSR(linear feedback shift register) used in B

Hi Neetin,

Yes these are decided by the equation.

-cheers
vlsi_eda_guy
 

Re: doubts on LFSR(linear feedback shift register) used in B

hi

akshay.raoy said:
use this document, it may be helpfull to u.
Click to expand...

thanks akshay......some of doubts got cleared from ur file............


regards

Neetin
 

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