[SOLVED] Doubt regarding the Cyclic Redundancy Check (CRC) in USB2.0

[sumannrao]

Hello Sir,

In protocol layer CRC (Cyclic Redundancy Check) is used to check errors.

The USB specification lists two generator polynomials - one for Tokens packet (ADDRESS + ENDPOINT = 11 bit field) and the
other for data packets(0 - 8192 bit field).
The generator polynomial for tokens is x5 + x2 + x0 while the generator polynomial for data packets is x16 + x15 + x2 + x0.

I would like to know is there any relationship in choosing the above Generator polynomial and the length of (Token/Data Packets).

also can we arbitrarily choose the generator polynomial?

Regards,
Suman

 

[yura717]

The protocol specification usually defines CRC in hex or polynomial notation. For example, CRC5 used in USB 2.0 protocol is represented as 0×5 in hex notation or as G(x)=x5+x2+1 in the polynomial notation. This CRC is typically implemented in hardware as a linear feedback shift register (LFSR) with a serial data input

 

[sumannrao]

Yes i agree the protocol specification usually defines CRC in hex or polynomial notation. For CRC5 used in USB 2.0 protocol is represented as 0×5 in hex notation or as G(x)=x5+x2+1 in the polynomial notation.

But my concern is what is the reason behind choosing the generator polynomial only as G(x)=x5+x2+1, and why it can't be something like this G(x)=x5+x3+1
or G(x)=x5+x3+x2+1 or any arbitrarily generator polynomial

Also I would like to know is there any relationship in choosing the above Generator polynomial and the length of (Token/Data Packets).

 

[yura717]

https://www.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf