Appendix A. Example C code of
the CCITT CRC
The
following
code
is
provided
as
an
example
for
programmers
implementing
their
own
code
to
communicate
with
the
sensor.
Users
using
Campbell
data
loggers
can
use
the
checksum
command
in
CRBasic
Editor
to
generate
a
CCITT
checksum.
Refer
to
the
downloadable
example
programs
available
at
/downloads/cs140-program-examples
.
The
checksum
includes
all
characters
after
the
STX
and
before
the
space
preceding
the
checksum.
The
SET
and
SETNC
commands
also
exclude
the
two
delimiting
:
characters,
one
on
each
side
of
the
checksum
itself.
//--------------------------------------------------------------- -------------
// Creates a CCITT CRC16 checksum seeded with 0x0000 (XModem style) using a
// fast non table based algorithm.
// Pass in the data to convert into a CRC in the form of a NULL terminated
// character array (a string).
// Returns the CRC in the form of an unsigned 16 bit integer value
// Note: This algorithm has only been tested on a native 16-bit processor with
// a hardware barrel shifter // All integers are 16-bits long
//--------------------------------------------------------------- -------------
unsigned int CRC_CCITT(char LineOfData[])
{
unsigned int crc;
// returned CRC value unsigned int i;
// counter crc = 0x0000;
// create a check sum for the incoming data for(i=0;i < strlen(LineOfData); i++)
{
unsigned crc_new = (unsigned char)(crc >> 8) | (crc << 8);
crc_new ^= LineOfData[i];
crc_new ^= (unsigned char)(crc_new & 0xff) >> 4;
crc_new ^= crc_new << 12;
crc_new ^= (crc_new & 0xff) << 5; crc = crc_new;
}
return(crc);
}
CS140 Background Luminance Sensor
34
