CRC in TMS37157

Dear Petri,

I am stuck just at the same point as you describe in your post, the exact calculation algorithm of the BCC for communication with the transponder.

It should be pretty straightforward, i mean, it loks like an standard CCITT CRC with 0x1021 polynom, 0x3791 initial value and calculation over the 7 bytes of data, excluding the 0x7E prolog. But there must be some subtlety with the implementation (or in the documentation) because i am totally unable to replicate its calculation.

Reading a more recent post of yours leads me to think that you might have done some progress with this. Could you provide any hint?

Best regards,

 Victor

Ok, I'll respond to myself.....

What you have to do is to implement __exactly__ the algorithm described in Figure52 of the datasheet, with initial value 0x3791.

Maybe it's just equivalent to the usual calculation but shifting bytes LSB-first... i dont' have time now to figure out....

Regards,

 Victor

Yes in my case the idea of standard CRC CCITT was abandoned and instead an exact implementation of the picture in the data sheet was done and it seems to work.

Hi,

   I'm stuck too. I tried web-tools, source code on internet on CCITT CRC, I even try to do my own program to calculate CRC (with information give by figure 51 of the datasheet). And I don't succeed to do anything right. And I don't really understand the figure 52 =(. Someone can help me, please ??

Thanks for the answers.

Matthias

Ok, I found  a solution =) :

My code :

"

/* CRC for TMS37157 */

#include<stdio.h>
#include<reg517.h>
#include<string.h>

int j=0;
int i=0;
int z=0;
char temp=2;
               
/*unsigned char tab[56]={'1', '1', '1', '1', '1', '1', '1', '1',         // (message = FF 01 0E 03 29 04 0E )
         '1', '0', '0', '0', '0', '0', '0', '0',                                                       // =>   CRC=A8 C0
         '0', '1', '1', '1', '0', '0', '0', '0',                         // bit to send FF 01 0E 03 29 04 0E C0 A8
         '1', '1', '0', '0', '0', '0', '0', '0',
         '1', '0', '0', '1', '0', '1', '0', '0',
         '0', '0', '1', '0', '0', '0', '0', '0',
         '0', '1', '1', '1', '0', '0', '0', '0'}; */

                 
unsigned char tab[48]={    '1', '0', '0', '1', '0', '1', '0', '0',                  //  (message = 29 05 04 03 02 01)
            '1', '0', '1', '0', '0', '0', '0', '0',                                                                 // =>   CRC=9E C2
            '0', '0', '1', '0', '0', '0', '0', '0',                                      // bit to send 29 05 04 03 02 01 C2 9E
            '1', '1', '0', '0', '0', '0', '0', '0',
            '0', '1', '0', '0', '0', '0', '0', '0',                           
            '1', '0', '0', '0', '0', '0', '0', '0'};    

/*unsigned char tab[56]={    '1', '0', '1', '1', '1', '1', '1', '0',                  // (message = 7D 04 00 00 00 00 00)              
                 '0', '0', '1', '0', '0', '0', '0', '0',                                                             // =>    CRC=AF 58
                 '0', '0', '0', '0', '0', '0', '0', '0',
                 '0', '0', '0', '0', '0', '0', '0', '0',                                              // bit to send 7D 04 00 00 00 00 00 58 AF
                 '0', '0', '0', '0', '0', '0', '0', '0',
                 '0', '0', '0', '0', '0', '0', '0', '0'};     */                         

unsigned char crc[16]={'0', '0', '1', '1', '0', '1', '1', '1',       // CRC with initial value 0x3791 in MSBF
         '1', '0', '0', '1', '0', '0', '0', '1'};   


unsigned char invert (unsigned char bit_invert)            // function to do invert a bit
{
if (bit_invert=='1')
    return('0');
else
    return('1');
}

void calc_CRC (unsigned char bit_data){        // function to modify the value of the CRC bit by bit
    temp=crc[15];                                                             // (based only on fig 52 of the TMS37157 (p.40))
    for (i=15;i>0;i--)        // crc = (crc >> 1);
        crc[i]=crc[i-1];                     
    crc[0]='0';                    
    if (bit_data=='1')
        crc[0]='1';
    else
        crc[0]='0';
    if(temp=='1')
        {
        crc[0]=invert(crc[0]);
        }
    if(crc[0]=='1')
        {
        crc[12]=invert(crc[12]);
        crc[5]=invert(crc[5]);
        }
}

void affiche_CRC (unsigned char * crc){            // function to display the CRC
    printf("\n ****   MSB: ");
for(j=0;j<4;j++)
    printf("%c",crc[j]);
printf(" ");
for(j=4;j<8;j++)
    printf("%c",crc[j]);
printf("  /  LSB: ");
for(j=8;j<12;j++)
    printf("%c",crc[j]);
printf(" ");
for(j=12;j<16;j++)
    printf("%c",crc[j]);
printf("\n\n\n");   
}

                          

int main (void)
{

printf("\n\n\n");

printf("  /// Valeur initial du CRC /// ");       
affiche_CRC(crc);                                // display initial value 0x3791

for(z=0;z<sizeof(tab);z++)
       calc_CRC(tab[z]);                        // modification of the CRC for all bit of the
                                                                      // message to send (for all the bit in "tab")
printf("  /// Valeur final du CRC /// ");        // final value
affiche_CRC(crc);                                // display the final value of the CRC

printf("fin du programme \n");

return (0);
}

"

this function calc_CRC (the key of the program) is only based on the figure 52 of the datasheet of the TMS37157 (pages 40).

Hope that it will help people ; ).

Matthias

fig 52 :