NXP Semiconductors LPC1768 - CRC Calculation

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User manual Rev. 3 — 19 December 2013 212 of 841
NXP Semiconductors
UM10360
Chapter 10: LPC176x/5x Ethernet
10.19 CRC calculation
The calculation is used for several purposes:
• Generation the FCS at the end of the Ethernet frame.
• Generation of the hash table index for the hash table filtering.
• Generation of the destination and source address hash CRCs.
The C pseudocode function below calculates the CRC on a frame taking the frame
(without FCS) and the number of bytes in the frame as arguments. The function returns
the CRC as a 32-bit integer.
int crc_calc(char frame_no_fcs[], int frame_len) {
int i; // iterator
int j; // another iterator
char byte; // current byte
int crc; // CRC result
int q0, q1, q2, q3; // temporary variables
crc = 0xFFFFFFFF;
for (i = 0; i < frame_len; i++) {
byte = *frame_no_fcs++;
for (j = 0; j < 2; j++) {
if (((crc >> 28) ^ (byte >> 3)) & 0x00000001) {
q3 = 0x04C11DB7;
} else {
q3 = 0x00000000;
}
if (((crc >> 29) ^ (byte >> 2)) & 0x00000001) {
q2 = 0x09823B6E;
} else {
q2 = 0x00000000;
}
if (((crc >> 30) ^ (byte >> 1)) & 0x00000001) {
q1 = 0x130476DC;
} else {
q1 = 0x00000000;
}
if (((crc >> 31) ^ (byte >> 0)) & 0x00000001) {
q0 = 0x2608EDB8;
} else {
q0 = 0x00000000;
}
crc = (crc << 4) ^ q3 ^ q2 ^ q1 ^ q0;
byte >>= 4;
}
}
return crc;
}
For FCS calculation, this function is passed a pointer to the first byte of the frame and the
length of the frame without the FCS.