16 bit CRC error checking for modbus

Modbus/inc/crc16.h

@@ -0,0 +1,88 @@
+/**
+@file
+CRC Computations
+
+@defgroup util_crc16 "util/crc16.h": CRC Computations
+@code#include "util/crc16.h"@endcode
+
+This header file provides functions for calculating
+cyclic redundancy checks (CRC) using common polynomials.
+Modified by Doc Walker to be processor-independent (removed inline
+assembler to allow it to compile on SAM3X8E processors).
+
+@par References:
+Jack Crenshaw's "Implementing CRCs" article in the January 1992 issue of @e
+Embedded @e Systems @e Programming. This may be difficult to find, but it
+explains CRC's in very clear and concise terms. Well worth the effort to
+obtain a copy.
+
+*/
+/* Copyright (c) 2002, 2003, 2004  Marek Michalkiewicz
+   Copyright (c) 2005, 2007 Joerg Wunsch
+   Copyright (c) 2013 Dave Hylands
+   Copyright (c) 2013 Frederic Nadeau
+   Copyright (c) 2015 Doc Walker
+   All rights reserved.
+
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions are met:
+
+   * Redistributions of source code must retain the above copyright
+     notice, this list of conditions and the following disclaimer.
+
+   * Redistributions in binary form must reproduce the above copyright
+     notice, this list of conditions and the following disclaimer in
+     the documentation and/or other materials provided with the
+     distribution.
+
+   * Neither the name of the copyright holders nor the names of
+     contributors may be used to endorse or promote products derived
+     from this software without specific prior written permission.
+
+  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+  POSSIBILITY OF SUCH DAMAGE. */
+
+
+#ifndef _UTIL_CRC16_H_
+#define _UTIL_CRC16_H_
+#include <chip.h>
+
+/** @ingroup util_crc16
+    Processor-independent CRC-16 calculation.
+
+    Polynomial: x^16 + x^15 + x^2 + 1 (0xA001)<br>
+    Initial value: 0xFFFF
+
+    This CRC is normally used in disk-drive controllers.
+
+    @param uint16_t crc (0x0000..0xFFFF)
+    @param uint8_t a (0x00..0xFF)
+    @return calculated CRC (0x0000..0xFFFF)
+*/
+static uint16_t crc16_update(uint16_t crc, uint8_t a)
+{
+  int i;
+
+  crc ^= a;
+  for (i = 0; i < 8; ++i)
+  {
+    if (crc & 1)
+      crc = (crc >> 1) ^ 0xA001;
+    else
+      crc = (crc >> 1);
+  }
+
+  return crc;
+}
+
+
+#endif /* _UTIL_CRC16_H_ */