2003-11-18  Theodore A. Roth  <address@hidden>

	* include/avr/crc16.h (_crc16_update): Add line break to dox.
	(_crc_xmodem_update): Ditto.
	* include/avr/crc16.h (_crc_ccitt_update): New function.
	[Submitted by Tomas Vanek <address@hidden>]

Index: include/avr/crc16.h
===================================================================
RCS file: /cvsroot/avr-libc/avr-libc/include/avr/crc16.h,v
retrieving revision 1.4
diff -u -p -r1.4 crc16.h
--- include/avr/crc16.h	11 Nov 2003 18:49:59 -0000	1.4
+++ include/avr/crc16.h	18 Nov 2003 18:41:42 -0000
@@ -54,7 +54,7 @@
 /** \ingroup avr_crc
     Optimized CRC-16 calcutation.
 
-    Polynomial: x^16 + x^15 + x^2 + 1 (0xa001)
+    Polynomial: x^16 + x^15 + x^2 + 1 (0xa001)<br>
     Initial value: 0xffff
 
     This CRC is normally used in disk-drive controllers. */
@@ -99,7 +99,7 @@ _crc16_update(uint16_t __crc, uint8_t __
 /** \ingroup avr_crc
     Optimized CRC-XMODEM calculation.
 
-    Polynomial: x^16 + x^12 + x^5 + 1 (0x1021)
+    Polynomial: x^16 + x^12 + x^5 + 1 (0x1021)<br>
     Initial value: 0x0
 
     This is the CRC used by the Xmodem-CRC protocol.
@@ -164,6 +164,72 @@ _crc_xmodem_update(uint16_t __crc, uint8
         "eor    %B0,%A0"         "\n\t" /* ret.hi is now ready. */
         "mov    %A0,%1"          "\n\t" /* ret.lo is now ready. */
         : "=d" (__ret), "=d" (__tmp1), "=d" (__tmp2)
+        : "r" (__data), "0" (__crc)
+        : "r0"
+    );
+    return __ret;
+}
+
+/** \ingroup avr_crc
+    Optimized CRC-CCITT calculation.
+
+    Polynomial: x^16 + x^12 + x^5 + 1 (0x8408)<br>
+    Initial value: 0xffff
+
+    This is the CRC used by PPP and IrDA.
+
+    See RFC1171 (PPP protocol) and IrDA IrLAP 1.1
+
+    \note Although the CCITT polynomial is the same as that used by the Xmodem
+    protocol, they are quite different. The difference is in how the bits are
+    shifted through the alorgithm. Xmodem shifts the MSB of the CRC and the
+    input first, while CCITT shifts the LSB of the CRC and the input first.
+
+    The following is the equivalent functionality written in C.
+
+    \code
+    uint16_t
+    crc_ccitt_update (uint16_t crc, uint8_t data)
+    {
+        data ^= lo8 (crc);
+        data ^= data << 4;
+
+        return ((((uint16_t)data << 8) | hi8 (crc)) ^ (uint8_t)(data >> 4) 
+                ^ ((uint16_t)data << 3));
+    }
+    \endcode */
+
+static inline uint16_t
+_crc_ccitt_update (uint16_t __crc, uint8_t __data)
+{
+    uint16_t __ret;
+
+    __asm__ (
+        "eor    %A0,%1"          "\n\t"
+
+        "mov    __tmp_reg__,%A0" "\n\t"
+        "swap   %A0"             "\n\t"
+        "andi   %A0,0xf0"        "\n\t"
+        "eor    %A0,__tmp_reg__" "\n\t"
+
+        "mov    __tmp_reg__,%B0" "\n\t"
+
+        "mov    %B0,%A0"         "\n\t"
+
+        "swap   %A0"             "\n\t"
+        "andi   %A0,0x0f"        "\n\t"
+        "eor    __tmp_reg__,%A0" "\n\t"
+
+        "lsr    %A0"             "\n\t"
+        "eor    %B0,%A0"         "\n\t"
+
+        "eor    %A0,%B0"         "\n\t"
+        "lsl    %A0"             "\n\t"
+        "lsl    %A0"             "\n\t"
+        "lsl    %A0"             "\n\t"
+        "eor    %A0,__tmp_reg__"
+
+        : "=d" (__ret)
         : "r" (__data), "0" (__crc)
         : "r0"
     );