A27系列优化I2C/RTC处理，新增版本A270Y

A27-STEPLDR/Src/crc32.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define CRC32_POLY 0x04C11DB7L
+ 
+unsigned int get_sum_poly(unsigned char top_byte)
+{
+    /// sum all the polys at various offsets 
+    unsigned int sum_poly = top_byte << 24;
+	int i;
+
+    for (i = 0; i < 8; ++i) {
+        /// check the top bit
+        if( ( sum_poly >> 31 ) != 0 )
+            /// TODO : understand why '<<' first
+            sum_poly = ( sum_poly << 1 ) ^ CRC32_POLY;
+        else
+            sum_poly <<= 1;
+    }
+ 
+    return sum_poly;
+}
+ 
+void create_table(unsigned int *table)
+{
+    for( int i = 0; i < 256; ++ i )
+    {
+        table[i] = get_sum_poly( (unsigned char) i );
+    }
+}
+
+static const unsigned int *crc32_table = NULL;
+
+unsigned int
+xcrc32(const unsigned char *buf, int len, unsigned int init)
+{
+	unsigned int crc = init;
+	static unsigned int table[256];
+
+	if (!crc32_table) {
+		create_table(table);
+		crc32_table = &table[0];
+	}
+	
+	while (len--)
+	{
+		crc = (crc << 8) ^ crc32_table[((crc >> 24) ^ *buf) & 255];
+		buf++;
+	}
+	return crc;
+}