# Drawing a CRC register from generating polynomial

#### derill03420

Joined Oct 15, 2011
26
Ok I need some help on this problem (the book i have for this online class is not explaining how the register is stuffed given the generator polynomial).

it says:

"Draw the CRC register for the generating polynomial x^5 + X^4 + x^2 + 1?

Now i think the register would be 6-bits and have bits 110101?

but i'm not sure and i am having trouble finding any good info

#### hgmjr

Joined Jan 28, 2005
9,029

#### Papabravo

Joined Feb 24, 2006
13,559
I think it would have five bits. The polynomial does not indicate a value to be stuffed into a register it indicates the locations of the feedback paths. If you set the original polynomial equal to 0, and exclusive or X^5 with both sides then you get something that makes a bit more sense
Rich (BB code):
X^5 + X^4 + X^2 + 1 = 0
x^5 = X^4 + x^2 + 1
What this polynomial means is that whenever a bit is is shifted out of the MSB ( the x^5) you exclusive or X^4 + X^2 + 1 with the 5-bit CRC register. If you multiply both sides of the equation by x you get
Rich (BB code):
X^6 = X^5 +X^3 + X
= X^4 + X^3 + X^2 + X + 1
Now you have a rule for doing the calculation 2 bits at a time instead of 1

#### derill03420

Joined Oct 15, 2011
26
Ok so what exactly am i being asked to draw when the question says " draw the CRC register for the generating polynomial"

I guess im not understanding what im supposed to be drawing, i was assuming i would be drawing an n-bit shift register with 1's and 0's in certain spots.

#### hgmjr

Joined Jan 28, 2005
9,029
I think the task is to design the shift register consistent with the given CRC equation.

hgmjr

#### derill03420

Joined Oct 15, 2011
26
ok maybe i get it now can someone check if i am on the right track?

so i think what i have to draw is a sort of modified shift register, where the terms in the generating polynomial are the feedback paths, and the terms that are missing in the generating polynomial are boxes connected together with no x-or gates between them. kind of like the link i listed here

http://en.wikipedia.org/wiki/File:CRC8-gen.gif

#### hgmjr

Joined Jan 28, 2005
9,029
That is way I would respond to the assignment.

hgmjr