📄 ccitt crc16 code.htm
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<HTML><HEAD><TITLE>CCITT CRC16 Code</TITLE>
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<P align=center><FONT size=7>S</FONT><FONT size=5>ample </FONT><FONT
size=7>CCITT CRC</FONT><FONT size=5> </FONT><FONT size=7>C</FONT><FONT
size=5>ode</FONT></P>
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</CENTER><PRE><FONT color=#ff0000>// Update the CRC for transmitted and received data using
// the CCITT 16bit algorithm (X^16 + X^12 + X^5 + 1).</FONT>
<B>unsigned</B> <B>char</B> ser_data;
<B>static unsigned</B> <B>int</B> crc;
crc = (<B>unsigned char</B>)(crc >> <FONT color=#0000ff>8</FONT>) | (crc << <FONT color=#0000ff>8</FONT>);
crc ^= ser_data;
crc ^= (<B>unsigned char</B>)(crc & <FONT color=#0000ff>0xff</FONT>) >> <FONT color=#0000ff>4</FONT>;
crc ^= (crc << <FONT color=#0000ff>8</FONT>) << <FONT color=#0000ff>4</FONT>;
crc ^= ((crc & <FONT color=#0000ff>0xff</FONT>) << <FONT color=#0000ff>4</FONT>) << <FONT color=#0000ff>1</FONT>;
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<P><I><FONT size=2><FONT color=#800080><B>Note:</B> It is best not to alter this
code. For example, </FONT><FONT
color=#008000>(crc<<8)<<4</FONT><FONT color=#800080> does not
generate the same code as </FONT><FONT color=#008000
size=2>crc<<12</FONT><FONT color=#800000 size=2>. </FONT><FONT
color=#800080 size=2>Although the result of the computation is the same, the
latter generates much more code and executes slower.</FONT></FONT></I>
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