Implementing CRC8 on Arduino to write to MLX90614

Question

UPDATE: I can't even get this calculator to reproduce the SMBus PECs illustrated in figures 8 and 9 of this datasheet!

So I'm interfacing an arduino with a Melexis temperature sensor, and it's going okay--aside from the fact that I can't seem to get the CRC check to work.

I've gotten read operations to complete successfully (although my software ignores the packet error code) but I have tried a lot of implementations of CRC8 to check the PEC byte to no avail. The code block I am using now came from OneWire:

uint8_t OneWire::crc8(const uint8_t *addr, uint8_t len)
{
    uint8_t crc = 0;

    while (len--) {
        uint8_t inbyte = *addr++;
        for (uint8_t i = 8; i; i--) {
            uint8_t mix = (crc ^ inbyte) & 0x01;
            crc >>= 1;
            if (mix) crc ^= 0x8C;
            inbyte >>= 1;
        }
    }
    return crc;
}

I rewrote it to consider just the one byte:

int smbCRC(int message) {

    uint8_t crc = 0;

  uint8_t inbyte = message & 0xFF;
  for (uint8_t i = 8; i; i--) {
    uint8_t mix = (crc ^ inbyte) & 0x01;
    crc >>= 1;
    if (mix) crc ^= 0x8C;
    inbyte >>= 1;
  }

    return crc;
}

But its CRC does not match that of the MLX datasheet (Figure 8 from here for example). When I print an int with its CRC8 like so:

int message = 0x3aD2;
lcd.print(String(message,HEX) + " " + String(smbCRC(message),HEX));

I get back "3ad2 eb", though the datasheet says the correct PEC is 0x30. Where am I going wrong? It seems like this could be caused by a bad implementation of CRC or bad assumptions on my part about the CRC input, and I'm not sure where to start troubleshooting.

Answer 1

I haven't checked your CRC implementation but there is a mistake in the MLX datasheet or at least it's badly written. You have to include all the I2C frame's data for the PEC's calculation not just the replied data. For a read word command you have to include [SA_W, Command, SA_R, LSB, MSB] and for a write word command [SA_W, Command, LSB, MSB].

So, for their first example the calculation must be made on [ 0xB4, 0x07, 0xB5, 0xD2, 0x3A ] and not just on [ 0xD2, 0x3A ] and this way you get the expected 0x30.

Here is a simple C implementation of the CRC with a lookup table (non Arduino but it must be quite simple to adapt):

static const uint8_t crc_table[] = {
    0x00, 0x07, 0x0e, 0x09, 0x1c, 0x1b, 0x12, 0x15, 0x38, 0x3f, 0x36, 0x31,
    0x24, 0x23, 0x2a, 0x2d, 0x70, 0x77, 0x7e, 0x79, 0x6c, 0x6b, 0x62, 0x65,
    0x48, 0x4f, 0x46, 0x41, 0x54, 0x53, 0x5a, 0x5d, 0xe0, 0xe7, 0xee, 0xe9,
    0xfc, 0xfb, 0xf2, 0xf5, 0xd8, 0xdf, 0xd6, 0xd1, 0xc4, 0xc3, 0xca, 0xcd,
    0x90, 0x97, 0x9e, 0x99, 0x8c, 0x8b, 0x82, 0x85, 0xa8, 0xaf, 0xa6, 0xa1,
    0xb4, 0xb3, 0xba, 0xbd, 0xc7, 0xc0, 0xc9, 0xce, 0xdb, 0xdc, 0xd5, 0xd2,
    0xff, 0xf8, 0xf1, 0xf6, 0xe3, 0xe4, 0xed, 0xea, 0xb7, 0xb0, 0xb9, 0xbe,
    0xab, 0xac, 0xa5, 0xa2, 0x8f, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9d, 0x9a,
    0x27, 0x20, 0x29, 0x2e, 0x3b, 0x3c, 0x35, 0x32, 0x1f, 0x18, 0x11, 0x16,
    0x03, 0x04, 0x0d, 0x0a, 0x57, 0x50, 0x59, 0x5e, 0x4b, 0x4c, 0x45, 0x42,
    0x6f, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7d, 0x7a, 0x89, 0x8e, 0x87, 0x80,
    0x95, 0x92, 0x9b, 0x9c, 0xb1, 0xb6, 0xbf, 0xb8, 0xad, 0xaa, 0xa3, 0xa4,
    0xf9, 0xfe, 0xf7, 0xf0, 0xe5, 0xe2, 0xeb, 0xec, 0xc1, 0xc6, 0xcf, 0xc8,
    0xdd, 0xda, 0xd3, 0xd4, 0x69, 0x6e, 0x67, 0x60, 0x75, 0x72, 0x7b, 0x7c,
    0x51, 0x56, 0x5f, 0x58, 0x4d, 0x4a, 0x43, 0x44, 0x19, 0x1e, 0x17, 0x10,
    0x05, 0x02, 0x0b, 0x0c, 0x21, 0x26, 0x2f, 0x28, 0x3d, 0x3a, 0x33, 0x34,
    0x4e, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5c, 0x5b, 0x76, 0x71, 0x78, 0x7f,
    0x6a, 0x6d, 0x64, 0x63, 0x3e, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2c, 0x2b,
    0x06, 0x01, 0x08, 0x0f, 0x1a, 0x1d, 0x14, 0x13, 0xae, 0xa9, 0xa0, 0xa7,
    0xb2, 0xb5, 0xbc, 0xbb, 0x96, 0x91, 0x98, 0x9f, 0x8a, 0x8d, 0x84, 0x83,
    0xde, 0xd9, 0xd0, 0xd7, 0xc2, 0xc5, 0xcc, 0xcb, 0xe6, 0xe1, 0xe8, 0xef,
    0xfa, 0xfd, 0xf4, 0xf3
};

uint8_t
crc8(uint8_t *p, uint8_t len)
{
        uint16_t i;
        uint16_t crc = 0x0;

        while (len--) {
                i = (crc ^ *p++) & 0xFF;
                crc = (crc_table[i] ^ (crc << 8)) & 0xFF;
        }

        return crc & 0xFF;
}

Answer 2

It is possible to get the compiler to figure out the lookup table for you, as follows:

#include <stdio.h>
#include <stdint.h>

/* * *
 * Just change this define to whatever polynomial is in use
 */
#define  CRC1B(b)       ( (uint8_t)((b)<<1) ^ ((b)&0x80? 0x07 : 0) ) // MS first

/* * *
 * 8+1 entry enum lookup table define
 */
#define  CRC(b)         CRC_##b          // or CRC8B(b)

enum {
    CRC(0x01) = CRC1B(0x80),
    CRC(0x02) = CRC1B(CRC(0x01)),
    CRC(0x04) = CRC1B(CRC(0x02)),
    CRC(0x08) = CRC1B(CRC(0x04)),
    CRC(0x10) = CRC1B(CRC(0x08)),
    CRC(0x20) = CRC1B(CRC(0x10)),
    CRC(0x40) = CRC1B(CRC(0x20)),
    CRC(0x80) = CRC1B(CRC(0x40)),
    // Add 0x03 to optimise in CRCTAB1
    CRC(0x03) = CRC(0x02)^CRC(0x01)
};

/* * *
 * Build a 256 byte CRC constant lookup table, built from from a reduced constant
 * lookup table, namely CRC of each bit, 0x00 to 0x80. These will be defined as
 * enumerations to take it easy on the compiler. This depends on the relation:
 *   CRC(a^b) = CRC(a)^CRC(b)
 * In other words, we can build up each byte CRC as the xor of the CRC of each bit.
 * So CRC(0x05) = CRC(0x04)^CRC(0x01). We include the CRC of 0x03 for a little more
 * optimisation, since CRCTAB1 can use it instead of CRC(0x01)^CRC(0x02), again a
 * little easier on the compiler.
 */

#define  CRCTAB1(ex)    CRC(0x01)ex, CRC(0x02)ex,  CRC(0x03)ex,
#define  CRCTAB2(ex)    CRCTAB1(ex)  CRC(0x04)ex,  CRCTAB1(^CRC(0x04)ex)
#define  CRCTAB3(ex)    CRCTAB2(ex)  CRC(0x08)ex,  CRCTAB2(^CRC(0x08)ex)
#define  CRCTAB4(ex)    CRCTAB3(ex)  CRC(0x10)ex,  CRCTAB3(^CRC(0x10)ex)
#define  CRCTAB5(ex)    CRCTAB4(ex)  CRC(0x20)ex,  CRCTAB4(^CRC(0x20)ex)
#define  CRCTAB6(ex)    CRCTAB5(ex)  CRC(0x40)ex,  CRCTAB5(^CRC(0x40)ex)

/* * *
 * This is the final lookup table. It is rough on the compiler, but generates the
 * required lookup table automagically at compile time.
 */
static const uint8_t crc_table[256] = { 0, CRCTAB6() CRC(0x80), CRCTAB6(^CRC(0x80)) };

uint8_t crc8(uint8_t *p, uint8_t len)
{
    uint8_t crc = 0x0;

    while (len--) {
        crc = crc_table[crc ^ *p++];
    }

    return crc;
}

void main( void )
{
    int i, j;

    printf("static const uint8_t crc_table[] = {");
    for (i = 0; i < 0x10; i++)
    {
        printf("\n   ");
        for (j = 0; j < 0x10; j++)
        {
            printf( " 0x%02x,", crc_table[i*0x10+j] );
        }
    }
    printf("\n};\n\n");
}

Answer 3

See reference on following links:

• http://www.melexis.com/Asset/SMBus-communication-with-MLX90614-DownloadLink-5207.aspx
• http://www.sbs-forum.org/marcom/dc2/20_crc-8_firmware_implementations.pdf

Function code for Arduino:

byte c8( byte x ){
  for( byte i = 8; i--; ) {
    x = ( x << 1 ) ^ ( x & 128 ? 7 : 0 );
  }
  return x;
}

void setup() {
  Serial.begin( 9600 );
  int msg = 0x3AD2;
  Serial.print( '0x' );
  Serial.print( c8( msg ), HEX );
  // '0x30' is displayed on Serial Monitor
}
void loop() {}

Since argument x is strong-typed as byte (uint8_t), word-size data (such 0x3AD2) will be truncated to byte-size (which is 0xD2 in case of 0x3AD2).