How to calculate the CRC in RFID protocol
문제
When reading data from a RFID device you will find a CRC-CCITT over the payload. "The CRC is initialized with 0x3791 instead of the usual value 0xFFFF." How can I define the function, that checks that the CRC is ok.
sample
data: { 0x02, 0x40, 0x00, 0x00, 0x00, 0x00, 0xA0 }
CRC: { 0x60, 0xE7 }
another sample
data: { 0x02, 0x41, 0x00, 0x00, 0x00, 0x00, 0xA4 }
CRC: { 0x6F, 0xA5 }
해결책
The only way I could get this to work was by implementing the bit-by-bit algorithm (TMS37157 datasheet Figure 52).
UINT16 rfid_get_crc(const UINT8 * data, INT8 size)
{
static BOOL lsb;
static BOOL rxdt;
static UINT16 crc;
static UINT8 bits;
static UINT8 byte;
static UINT8 i;
const UINT16 RFID_CRC_INIT = 0x3791;
crc = RFID_CRC_INIT;
for (i=0; i<size; i++)
{
bits = 8;
byte = data[i]; // Next byte
while (bits --> 0)
{
lsb = crc & 1; // Store LSB
crc >>= 1; // Shift right 1 bit
rxdt = byte & 1;
if (rxdt)
crc |= 0x8000; // Shift in next bit
if (lsb) // Check stored LSB
crc ^= 0x8000; // Invert MSB
if (0x8000 == (crc & 0x8000)) // Check MSB
crc ^= 0x0408; // Invert bits 3 and 10
byte >>= 1; // Next bit
}
}
return crc;
}
다른 팁
A more compact implementation( in pseudo code ) is:
// Least significant bit first (little-endian)
// x^16+x^12+x^5+1 = 1000 0100 0000 1000 (1) = 0x8408
function crc(byte array string[1..len], int len) {
//Other RFID tags I have seen use initialization of 0x0000:
//rem := 0x3791;
rem := 0x3791;
for i from 1 to len {
rem := rem xor string[i]
for j from 1 to 8 { // Assuming 8 bits per byte
if rem and 0x0001 { // if rightmost (most significant) bit is set
rem := (rem rightShift 1) xor 0x8408
} else {
rem := rem rightShift 1
}
}
}
// A popular variant complements rem here
return rem
This can be found in Code Fragment 5 of:
http://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks
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