add fast CRC8 computation

readme.md

@@ -9,8 +9,9 @@ Create formatted frame from basic types:
 
 Structure of data frame:
   - starting byte (0xCC)
-  - length of frame: count of bytes in payload. Maximum length: 254 (0xFE)h
+  - length of frame: count of bytes in payload. The first 2 bytes is not included to overal length. Maximum length: 254 (0xFE)
   - payload
+  - CRC8
  
    
 
@@ -32,4 +33,9 @@ Structure of data frame:
     int data_length = packet_commit();
     }
 
-```
+```
+
+## CRC8 Computation
+
+- Generator polynome: 0x97 ->  0x197 = x^8 + x^7 + x^4 + x^2 + x^1 +1 
+- CRC is implemented as fast computation with prepared CrcTable. Algorithm complexity is O(n)

src/dataframe.c

@@ -7,9 +7,10 @@
  *  @brief Simple Data Frame
  *
  *  Structure of data frame:
- *  - starting byte (0xCC)
- *  - length of frame: count of bytes in payload. Maximum length: 254 (0xFE)h
- *  - payload
+ * - starting byte (0xCC)
+ * - length of frame: count of bytes in payload. The first 2 bytes is not included to overal length. Maximum length: 254 (0xFE)
+ * - payload
+ * - CRC8
  *
  *  Supported data types:
  *  - uint8_t, int8_t
@@ -40,6 +41,53 @@ uint8_t *packet;
 static volatile uint8_t length;
 static volatile uint8_t capacity;
 
+uint8_t crc8(uint8_t crc, uint8_t Size, uint8_t *Buffer)
+{
+    // https://community.st.com/s/question/0D50X0000CDmAkpSQF/calculate-crc8
+    static const unsigned char CrcTable[] = { // 0x97 Polynomial Table, 8-bit,
+            0x00,0x97,0xB9,0x2E,0xE5,0x72,0x5C,0xCB,
+            0x5D,0xCA,0xE4,0x73,0xB8,0x2F,0x01,0x96,
+            0xBA,0x2D,0x03,0x94,0x5F,0xC8,0xE6,0x71,
+            0xE7,0x70,0x5E,0xC9,0x02,0x95,0xBB,0x2C,
+            0xE3,0x74,0x5A,0xCD,0x06,0x91,0xBF,0x28,
+            0xBE,0x29,0x07,0x90,0x5B,0xCC,0xE2,0x75,
+            0x59,0xCE,0xE0,0x77,0xBC,0x2B,0x05,0x92,
+            0x04,0x93,0xBD,0x2A,0xE1,0x76,0x58,0xCF,
+            0x51,0xC6,0xE8,0x7F,0xB4,0x23,0x0D,0x9A,
+            0x0C,0x9B,0xB5,0x22,0xE9,0x7E,0x50,0xC7,
+            0xEB,0x7C,0x52,0xC5,0x0E,0x99,0xB7,0x20,
+            0xB6,0x21,0x0F,0x98,0x53,0xC4,0xEA,0x7D,
+            0xB2,0x25,0x0B,0x9C,0x57,0xC0,0xEE,0x79,
+            0xEF,0x78,0x56,0xC1,0x0A,0x9D,0xB3,0x24,
+            0x08,0x9F,0xB1,0x26,0xED,0x7A,0x54,0xC3,
+            0x55,0xC2,0xEC,0x7B,0xB0,0x27,0x09,0x9E,
+            0xA2,0x35,0x1B,0x8C,0x47,0xD0,0xFE,0x69,
+            0xFF,0x68,0x46,0xD1,0x1A,0x8D,0xA3,0x34,
+            0x18,0x8F,0xA1,0x36,0xFD,0x6A,0x44,0xD3,
+            0x45,0xD2,0xFC,0x6B,0xA0,0x37,0x19,0x8E,
+            0x41,0xD6,0xF8,0x6F,0xA4,0x33,0x1D,0x8A,
+            0x1C,0x8B,0xA5,0x32,0xF9,0x6E,0x40,0xD7,
+            0xFB,0x6C,0x42,0xD5,0x1E,0x89,0xA7,0x30,
+            0xA6,0x31,0x1F,0x88,0x43,0xD4,0xFA,0x6D,
+            0xF3,0x64,0x4A,0xDD,0x16,0x81,0xAF,0x38,
+            0xAE,0x39,0x17,0x80,0x4B,0xDC,0xF2,0x65,
+            0x49,0xDE,0xF0,0x67,0xAC,0x3B,0x15,0x82,
+            0x14,0x83,0xAD,0x3A,0xF1,0x66,0x48,0xDF,
+            0x10,0x87,0xA9,0x3E,0xF5,0x62,0x4C,0xDB,
+            0x4D,0xDA,0xF4,0x63,0xA8,0x3F,0x11,0x86,
+            0xAA,0x3D,0x13,0x84,0x4F,0xD8,0xF6,0x61,
+            0xF7,0x60,0x4E,0xD9,0x12,0x85,0xAB,0x3C };
+
+    while(Size--)
+    {
+        crc = crc ^ *Buffer++; // Apply Byte
+        crc = CrcTable[crc & 0xFF]; // One round of 8-bits
+    }
+
+    return(crc);
+}
+
+
 void packet_create(uint8_t *usr_packet, uint8_t size) {
     packet = usr_packet;
     capacity = size;
@@ -77,10 +125,10 @@ uint8_t packet_add_uint32(uint32_t d)
     if((length+4) >= capacity){
         return VALUE_ERR;
     }
-	packet[length++] =  (d >> 24) & 0xFF;
-	packet[length++] =  (d >> 16) & 0xFF;
-	packet[length++] =  (d >> 8) & 0xFF;
 	packet[length++] =  d & 0xFF;
+	packet[length++] = (d >> 8) & 0xFF;
+	packet[length++] = (d >> 16) & 0xFF;
+	packet[length++] = (d >> 24) & 0xFF;
     return VALUE_OK;
 }
 
@@ -118,6 +166,10 @@ uint8_t packet_add_float(float f)
 
 uint8_t packet_commit()
 {
-	packet[1] = length;
+	// compute CRC
+	packet[length] = crc8(0, length-2, &(*(packet+2)));
+	packet[1] = length - 1;
+	// length of payload: from byte 2 to CRC byte (the last)
 	return length;
+
 }