From b0f5c3e9b66c45e1c56c27cf10e78bfd0364aa0a Mon Sep 17 00:00:00 2001
From: Stefan `Sec` Zehl <sec@42.org>
Date: Fri, 8 Jul 2011 00:10:53 +0200
Subject: [PATCH] add openbeacon-compatible crc (CCITT-16)

---
 firmware/Makefile      |  3 ++-
 firmware/basic/basic.h |  4 ++++
 firmware/basic/crc.c   | 24 ++++++++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 firmware/basic/crc.c

diff --git a/firmware/Makefile b/firmware/Makefile
index 65dbcf0..af2f8ef 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -8,7 +8,7 @@ OBJS = main.o
 VPATH +=
 OBJS +=
 OBJS += basic/basic.o basic/reinvoke_isp.o basic/delayms.o basic/voltage.o
-OBJS += basic/keyin.o basic/uuid.o
+OBJS += basic/keyin.o basic/uuid.o basic/crc.o
 LIBS += core/libcore.a lcd/liblcd.a applications/libapp.a filesystem/libfat.a usb/libusb.a funk/libfunk.a
 
 ##########################################################################
@@ -106,3 +106,4 @@ $(OUTFILE).elf: $(OBJS) $(SYS_OBJS) $(LIBS) $(LPCFIX) $(LD_TEMP)
 
 .PHONY: $(LD_TEMP) lcd/liblcd.a applications/libapp.a filesystem/libfat.a usb/libusb.a
 
+
diff --git a/firmware/basic/basic.h b/firmware/basic/basic.h
index 4bd93ea..ba942af 100644
--- a/firmware/basic/basic.h
+++ b/firmware/basic/basic.h
@@ -152,4 +152,8 @@ uint16_t GetUUID16(void);
 // for core/iap/iap.c (no official definition)
 void iap_entry(uint32_t param_tab[], uint32_t result_tab[]);
 
+// crc.c
+uint16_t crc16(char * buf, int len);
+
+
 #endif
diff --git a/firmware/basic/crc.c b/firmware/basic/crc.c
new file mode 100644
index 0000000..9846888
--- /dev/null
+++ b/firmware/basic/crc.c
@@ -0,0 +1,24 @@
+#include "basic.h"
+
+// Calculate the CRC for transmitted and received data using
+// the CCITT 16bit algorithm (X^16 + X^12 + X^5 + 1).
+
+uint16_t crc16(char * buf, int len){
+    unsigned int crc=0xffff;
+
+	for(int i=0;i<len;i++){
+		crc  = (unsigned char)(crc >> 8) | (crc << 8);
+		crc ^= buf[i];
+		crc ^= (unsigned char)(crc & 0xff) >> 4;
+		crc ^= (crc << 8) << 4;
+		crc ^= ((crc & 0xff) << 4) << 1;
+	};
+	return crc;
+};
+
+/* Note:
+   It is best not to alter this code. For example, (crc<<8)<<4 does
+   not generate the same code as crc<<12. Although the result of the
+   computation is the same, the latter generates much more code and
+   executes slower.
+ */