CRC for eeprom data check

Question

I wanted to check redundancy in eeprom and crosscheck the data for any errors occurred or not and I ended up reading CRC code. I found it useful but have some doubts that need clarity.

My objective is to check the memory address locations 0-15k of eeprom and store the checksum in memory and do the same for next 15k-30k address locations. The Both address locations store the same data and it is done so that eeprom memory is utilized.

Well When I began reading the docs. I ended up with a useful code:

unsigned int crc32(unsigned char *message) {
  int i, j;
  unsigned int byte, crc;
  i = 0;
  crc = 0xFFFFFFFF;
  while (message[i] != 0) {
    byte = message[i]; // Get next byte.
    byte = reverse(byte); // 32-bit reversal.
    for (j = 0; j <= 7; j++) { // Do eight times.
      if ((int)(crc ^ byte) < 0)
        crc = (crc << 1) ^ 0x04C11DB7;
      else crc = crc << 1;
      byte = byte << 1; // Ready next msg bit.
    }
    i = i + 1;
  }
  return reverse(~crc);
}

Here as far as my understanding the 'message' awaits bytes and continues CRC calculation until the character is not received. If I wanted to check the two blocks of eeprom memory as said before. How can I append the eeprom memory to the message variable.

Well my another *doubt is whether eeprom read have any limit regarding reading reading address from 0-15k at a stretch rather like eeprom write * I need your inputs to make this attempt useful and rewarding Regards

Answer 1

While the suggested approach from PeterJ is fine, its cleaner to decouple the "data layer" logic (EEPROM access) from the CRC routine.

This CRC-32 can be universally used, its not bound to program specific behavior:

// CCITT CRC-32 (Autodin II) polynomial
uint32_t CalcCRC32(uint32_t crc, uint8_t *buffer, uint16_t length) {
    while(length--) {
        crc = crc ^ *buffer++; 

        for (uint8_t j=0; j < 8; j++) { 
           if (crc & 1) 
              crc = (crc >> 1) ^ 0xEDB88320; 
           else 
              crc = crc >> 1; 
        } 
    } 

   return crc; 
}

Then you just feed blocks of data to the CRC function like you see fit and as required by your application.

This usage example is just written down. It uses the fictional function eeprom_read() which reads a block of data from EEPROM. We start at EEPROM address 0.

const uint8_t BLOCKLENGTH = 128;
uint32_t crc32;
uint8_t buffer[BLOCKLENGTH];    // temporary data buffer

crc32 = 0xFFFFFFFF;  // initial CRC value 

for (uint16_t i=0; i < NUMBEROFBLOCKS; i++) {
    eeprom_read(BLOCKLENGTH * i, buffer, BLOCKLENGTH);    // read block number i from EEPROM into buffer
    crc32 = CalcCRC32(crc32, buffer, BLOCKLENGTH);        // update CRC
}

// crc32 is your final CRC here

Note that NUMBEROFBLOCKS is just a placeholder. I hope you get the idea.

Answer 2

If you're using a microcontroller with limited RAM you probably won't want to try loading a 16k chunk of data into memory. Re-arranging the code above you could try the below (untested) function that reads the EEPROM byte by byte within the loop. The function read_eeprom is something I've made up so you'll need to find the appropriate function for your compiler.

#include <stdint.h>

uint32_t crc32(uint16_t start_address, uint16_t number_of_bytes) {
  uint8_t j;
  uint32_t crc, byte;
  crc = 0xFFFFFFFF;
  while (number_of_bytes--) {
    byte = read_eeprom(start_address++);
    byte = reverse(byte); // 32-bit reversal.
    for (j = 0; j <= 7; j++) { // Do eight times.
      if ((int32_t)(crc ^ byte) < 0)
        crc = (crc << 1) ^ 0x04C11DB7;
      else 
        crc = crc << 1;
      byte = byte << 1; // Ready next msg bit.
    }
  }
  return reverse(~crc);
}

I've also replaced your variable declarations to use stdint.h - it's probably good to read about that and use it. On some embedded compilers an unsigned int won't be 32 bits for example so this makes the code more portable.