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LFSR simulation in Hspice

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020170

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hspice lfsr

LFSR is "linear feedback shift register"

It is used to make pseudo random generator

I'm afraid that this circuit is not working properly.

all D output digital number is 0, it does not generate some codes

in additional to, in my thought, It does not generate random codes, just shift codes
from left to right.

When I simulate this circuit, is there any thing I have to concern ?

thanks
 

hspice prbs

You must set a non-zero seed, otherwise you will get all '0's.
I use it to generate prbs data. There's no problem.

If you need random data, suggest you to generate by program such as matlab, C...
 

lfsr hspice

thanks for your reply.

I need prbs data but I'll get it by using hardware stuff like LFSR

How do I insert initial seed into LFSR?

I have to use modified LFSR like De Bruijn counter ?


could you tell me where did you find it if you have some paper or document?

thanks

Another Question:

LFSR is not truly ramdom signal, Pseudo random.

But it doesn't look like pseudo random number. see the following picture

it have about 1.0m period.

what do I do to get enough long period?

Thanks
 

hspice prbs

"hspice simulation and analysis user guide" v2005.09 P190

lxxx n+ n- LFSR <(> vlow vhigh tdelay trise tfall rate seed <[>
+ taps <]> <rout=val> <)>

For example this is a prbs7 with seed of all 1's.
V1 n1 0 LFSR (0 1 td tr tr datarate 'pow(2,7)-1' [7, 6] )
 

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    020170

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lfsr pspice

Just u initialize one d ff o/p to 1 initially and then make it to work in normal operation
 

prbs7 lfsr

For the initial seed u can add logic that transfer u from state 0000 to 0001 for example ,and for long period u have to increase the number of FFs used
 

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