On Wed, 24 Mar 2004 21:25:03 -0700, Dan Henry <dhenry@sprynet.com>
wrote:

>If you want it, I guess you'll have to divulge your e-mail address so
>I can send it to you, as the one given suggests that it's a fake and
>that you don't have spam filters.

Thanks Dan,

dan_nelson123  at  hotmail.com

Dan

"Bob Stephens" <stephensyomamadigital@earthlink.net> wrote in message
news:ct7oxhusnjye.1sekmxocmcbtu.dlg@40tude.net...
> On Thu, 25 Mar 2004 13:48:43 GMT, Rick Merrill wrote:
>
> > you might prefer
> >> unsigned char calcCrc(void *data, unsigned int size) {
> >>     unsigned char crc = 0;
> >>     while (0<size--) {
> >>         crc = crcTable[crc^(*data)++];
> >>     };
> >>     return crc;
> >> }
> > [++ has higher precedence than *]
>
> Huh?

For some reason, I don't seem to have got Rick's post on my newserver...

Yes, ++ has higher precedence than * .  Normally I like to add extra
brackets to make these things clearer, but expressions of the form *p++ are
so commonly used that I didn't bother.  The original expression was

    crc = crcTable[crc ^ *data++]

which means

    crc = crcTable[crc ^ ( * (data++) ) ]

The expression (*data)++ would mean something else entirely - it would
increment the value at address "data" instead - what I am doing here is
running through "size" bytes at address "data" onwards.

On Thu, 25 Mar 2004 13:48:43 GMT, Rick Merrill wrote:

> you might prefer
>> unsigned char calcCrc(void *data, unsigned int size) {
>>     unsigned char crc = 0;
>>     while (0<size--) {
>>         crc = crcTable[crc^(*data)++];
>>     };
>>     return crc;
>> }
> [++ has higher precedence than *]

Huh?

you might prefer
> unsigned char calcCrc(void *data, unsigned int size) {
>     unsigned char crc = 0;
>     while (0<size--) {
>         crc = crcTable[crc^(*data)++];
>     };
>     return crc;
> }
[++ has higher precedence than *]

There is a very popular paper by Ross N. Williams entitled "A Painless Guide
to CRC Error Detection Algorithms", which is available at dozens of web
sites, such as:
http://www.repairfaq.org/filipg/LINK/F_crc_v3.html

It is definitely worth reading before you get in too deep.  By far the
easiest and fastest method of implementing crc checks is to use a lookup
table (at the cost of 256 bytes of program memory).  Once you have generated
your table, the crc check function is something like:

unsigned char calcCrc(void *data, unsigned int size) {
    unsigned char crc = 0;
    while (size--) {
        crc = crcTable[crc ^ *data++];
    };
    return crc;
}




"Dan" <dan@dontspammecauseidontlikit.com> wrote in message
news:tvf460h6phqol5emtte8t8r1u331ed33ua@4ax.com...
>
> I'm trying to implement the mux protocol (GSM 07.10) with a Motorola
> G18 modem.  I have the spec ETSI TS 101 369 which describes the CRC
> as:
>
> The FCS shall be the ones complement of the sum (modulo 2) of
> a) the remainder of
> xk (x7 + x6 + x5 + x4 + x3 + x2 + x1 + 1)
> divided (modulo 2) by the generator polynomial
> x8 + x2 + x + 1,
> etcetera...
>
> What I really want is the C code that implements the algorithm.  I've
> found some examples on the web, but they're 16 bit or 32 bit, whereas
> this algorithm is 8 bit.
>
> I know that if I perservere, I can figure out how to implement it, but
> I can see a lot of unsuccessful attempts along the way.  Does anyone
> have C code for this?
>
> Thanks
>
> Dan

On Thu, 25 Mar 2004 10:20:15 +0800, Dan
<dan@dontspammecauseidontlikit.com> wrote:

>
>I'm trying to implement the mux protocol (GSM 07.10) with a Motorola
>G18 modem.  I have the spec ETSI TS 101 369 which describes the CRC
>as:
>
>The FCS shall be the ones complement of the sum (modulo 2) of
>a) the remainder of
>xk (x7 + x6 + x5 + x4 + x3 + x2 + x1 + 1)
>divided (modulo 2) by the generator polynomial
>x8 + x2 + x + 1,
>etcetera...
>
>What I really want is the C code that implements the algorithm.  I've
>found some examples on the web, but they're 16 bit or 32 bit, whereas
>this algorithm is 8 bit.
>
>I know that if I perservere, I can figure out how to implement it, but
>I can see a lot of unsuccessful attempts along the way.  Does anyone
>have C code for this?
>
>Thanks
>
>Dan

I have what you are looking for, but posting 500 lines of source code
here would be frowned upon -- 500 lines because of heavy commenting
and multiple implementations to select from via conditional
compilation paths, not because the CRC is difficult to implement.

If you want it, I guess you'll have to divulge your e-mail address so
I can send it to you, as the one given suggests that it's a fake and
that you don't have spam filters.

--
Dan Henry

You might try grabbing a copy of "Programming Embedded Systems in C and C++"
from O'Reilly. It has a short section showing how CRCs are described and has
code you can modify to fit whatever parameters are given. I haven't used it
though. The parameters are the width, polynomial, initial remainder, and final
xor value.

I'm trying to implement the mux protocol (GSM 07.10) with a Motorola
G18 modem.  I have the spec ETSI TS 101 369 which describes the CRC
as:

The FCS shall be the ones complement of the sum (modulo 2) of
a) the remainder of
xk (x7 + x6 + x5 + x4 + x3 + x2 + x1 + 1)
divided (modulo 2) by the generator polynomial
x8 + x2 + x + 1,
etcetera...

What I really want is the C code that implements the algorithm.  I've
found some examples on the web, but they're 16 bit or 32 bit, whereas
this algorithm is 8 bit.

I know that if I perservere, I can figure out how to implement it, but
I can see a lot of unsuccessful attempts along the way.  Does anyone
have C code for this?

Thanks

Dan