C8051F80X_14 Datasheet, PDF (161/251 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F80x-83x
24.2. 32-bit CRC Algorithm
The C8051F80x-83x CRC unit calculates the 32-bit CRC using a poly of 0x04C11DB7. The CRC-32 algo-
rithm is "reflected", meaning that all of the input bytes and the final 32-bit output are bit-reversed in the pro-
cessing engine. The following is a description of a simplified CRC algorithm that produces results identical
to the hardware:
1. XOR the least-significant byte of the current CRC result with the input byte. If this is the first iteration of
the CRC unit, the current CRC result will be the set initial value (0x00000000 or 0xFFFFFFFF).
2. Right-shift the CRC result.
3. If the LSB of the CRC result is set, XOR the CRC result with the reflected polynomial (0xEDB88320).
4. Repeat at Step 2 for the number of input bits (8).
For example, the 32-bit C8051F80x-83x CRC algorithm can be described by the following code:
unsigned long UpdateCRC (unsigned long CRC_acc, unsigned char CRC_input){
unsigned char i; // loop counter
#define POLY 0xEDB88320 // bit-reversed version of the poly 0x04C11DB7
// Create the CRC "dividend" for polynomial arithmetic (binary arithmetic
// with no carries)
CRC_acc = CRC_acc ^ CRC_input;
// "Divide" the poly into the dividend using CRC XOR subtraction
// CRC_acc holds the "remainder" of each divide
// Only complete this division for 8 bits since input is 1 byte
for (i = 0; i < 8; i++)
{
// Check if the MSB is set (if MSB is 1, then the POLY can "divide"
// into the "dividend")
if ((CRC_acc & 0x00000001) == 0x00000001)
{
// if so, shift the CRC value, and XOR "subtract" the poly
CRC_acc = CRC_acc >> 1;
CRC_acc ^= POLY;
}
else
{
// if not, just shift the CRC value
CRC_acc = CRC_acc >> 1;
}
}
return CRC_acc; // Return the final remainder (CRC value)
}
Table 24.2 lists example input values and the associated outputs using the 32-bit C8051F80x-83x CRC
algorithm (an initial value of 0xFFFFFFFF is used):
Table 24.2. Example 32-bit CRC Outputs
Input
0x63
0xAA, 0xBB, 0xCC
0x00, 0x00, 0xAA, 0xBB, 0xCC
Output
0xF9462090
0x41B207B3
0x78D129BC
Rev. 1.0
161