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[v1-02] 2018-Feb-19

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ENS210 − Functional Description

Computing CRC-7

CRC algorithm uses a 7 bit polynomial (see lines 4, 5, and 6), and

a 17 bit payload. The crc7() function below uses the following

constants defining the CRC width, (the coefficients of the)

polynomial and the initial vector (start value of the CRC), and

some constants describing the payload data size.

7654

3210

// Polynomial

1000

1001 ~ x^7+x^3+x^0

#define CRC7WIDTH

// 7 bits CRC has polynomial of 7th order (has 8 terms)

#define CRC7POLY

0x89

// The 8 coefficients of the polynomial

#define CRC7IVEC

0x7F

// Initial vector has all 7 bits high

// Payload data

#define DATA7WIDTH 17

#define DATA7MASK

((1UL<<DATA7WIDTH)-1) // 0b 0 1111 1111 1111 1111

#define DATA7MSB

(1UL<<(DATA7WIDTH-1)) // 0b 1 0000 0000 0000 0000

The crc7(val) function returns the CRC-7 of a 17 bits value val.

// Compute the CRC-7 of 'val' (should only have 17 bits)

uint32_t crc7( uint32_t val ) {

// Setup polynomial

uint32_t pol= CRC7POLY;

// Align polynomial with data

pol = pol << (DATA7WIDTH-CRC7WIDTH-1);

// Loop variable (indicates which bit to test, start with highest)

uint32_t bit = DATA7MSB;

// Make room for CRC value

val = val << CRC7WIDTH;

bit = bit << CRC7WIDTH;

pol = pol << CRC7WIDTH;

// Insert initial vector

val |= CRC7IVEC;

// Apply division until all bits done

while( bit & (DATA7MASK<<CRC7WIDTH) ) {

if( bit & val ) val ^= pol;

bit >>= 1;

pol >>= 1;

}

return val;

}

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