Pololu Simple - Manual (Page 99)

6.7.6. CRC Computation in C
Simple Example
The following example program shows how to compute a CRC byte in the C language. The outer loop processes each
byte, and the inner loop processes each bit of those bytes. In the example main() routine, this is applied to generate
the CRC byte in the message 0x83, 0x01, that was used in
Section 6.5 . The
getCRC()
function will work without
modification in both Arduino and Orangutan programs.
const unsigned char CRC7_POLY = 0x91;
unsigned char getCRC(unsigned char message[], unsigned char length)
{
unsigned char i, j, crc = 0;
for (i = 0; i < length; i++)
{
crc ^= message[i];
for (j = 0; j < 8; j++)
{
if (crc & 1)
crc ^= CRC7_POLY;
crc >>= 1;
}
}
return crc;
}
int main()
{
// create a message array that has one extra byte to hold the CRC:
unsigned char message[3] = {0x83, 0x01, 0x00};
message[2] = getCRC(message, 2);
// send this message to the Simple Motor Controller
}
Advanced Example
The following example program shows a more efficient way to compute a CRC in the C language. The increased
efficiency is achieved by pre-computing the CRCs of all 256 possible bytes and storing them in a lookup table, which
can be in RAM, flash, or EEPROM. These table values are then XORed together based on the bytes of the message to
get the final CRC. In the example main() routine, this is applied to generate the CRC byte in the message 0x83, 0x01,
that was used in
Section 6.5 .
#include <stdio.h>
const unsigned char CRC7_POLY = 0x91;
unsigned char CRCTable[256];
unsigned char getCRCForByte(unsigned char val)
{
unsigned char j;
for (j = 0; j < 8; j++)
{
if (val & 1)
val ^= CRC7_POLY;
val >>= 1;
}
return val;
}
void buildCRCTable()
{
int i;
// fill an array with CRC values of all 256 possible bytes
for (i = 0; i < 256; i++)
{
Pololu Simple Motor Controller User's Guide © 2001–2014 Pololu Corporation
6. Using the Serial Interface Page 99 of 101