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16 bit crc

Discussion in 'Electronic Design' started by Sreenath, Dec 29, 2006.

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  1. Sreenath

    Sreenath Guest

    ---------------- |----------------|--------------|
    ^ v v v
    ip-->(+)<- |x15 ..x12|<-(+)<-|x11.. x5 |<-(+)<-|x4 .. x0|<---

    ---------------- |---------------|-----------------|
    ^ v v v
    <- |x15 ..x12|<-(+)<-|x11.. x5 |<-(+)<-|x4 .. x0|<---(+)<-ip

    What is the difference between the 2 types of CRC calcultion
    mentioned above.
    In the fisrt one input is xored with x15 and fed at different points
    while in the second one the ip is xored and fed at the fisrt point.
    I understand that second one computes the crc or remainder for
    the pattern of ip but what does the first one compute.

    (how to send diagrams .it would have been esay to draw diagram)

  2. HKJ

    HKJ Guest

    Is it reflected and non-reflected?
    Correctly used they will give the same result.
  3. Ben Jackson

    Ben Jackson Guest

    In written form you could just use the CRC polynomial, such as
    CRC-16: x^16 + x^15 + x^2 + 1

    I couldn't quite read your diagram, but perhaps you are describing
    a reciprocal polynomial, which has all of the coefficients reversed.
    It's just as strong as the original but the output has the reverse
    bit order. From my example, it'd be x^16 + x^14 + x^1 + 1.
  4. jasen

    jasen Guest

    only the way the internal state is represented.
    the CRCs produced at out should be the same...

    your two: (use fixed font)

    | | | |
    ^ v v |
    ip-->(+)<-+--|x15 ..x12|<-(+)<-|x11.. x5 |<-(+)<-|x4 .. x0|<-'
    `---> out

    .----------------+-----------------+------------------+----> out
    | | | |
    | v v v
    `- |x15 ..x12|<-(+)<-|x11.. x5 |<-(+)<-|x4 .. x0|<---(+)<-ip

    another way:

    .-------------->(+)------------->(+)----------------+----> out
    | ^ ^ |
    | | | v
    `- |x15 ..x12|<--+--|x11.. x5 |<--+--|x4 .. x0|<---(+)<-ip

    for simple diagrams like above uses a fixed pitch font and plenty of
    whitespace. more complex drawings can be posted to
    alt.binaries.schematics.electronics in some common format like gif or pdf etc...

  5. Sreenath

    Sreenath Guest

    The out is genrated with every ip but what we need is 16 bit for the
    whole of the input.
    the remainder or the 16 bit CRC.
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