Adds CRC checksum validation to LJ1200 messages (using Eric Cottrell's CRC polynomial)

2026-11-12 08:37:25 -05:00 · 2014-12-05 12:59:16 +00:00
parent 11c52cbe19
commit 412f1ac9f7
3 changed files with 240 additions and 39 deletions
--- a/src/crc/CRCLJ.java
+++ b/src/crc/CRCLJ.java
@@ -0,0 +1,205 @@
+/*******************************************************************************
+ *     SDR Trunk 
+ *     Copyright (C) 2014 Dennis Sheirer
+ * 
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ * 
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ * 
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <http://www.gnu.org/licenses/>
+ ******************************************************************************/
+package crc;
+
+import java.util.BitSet;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import bits.BitSetBuffer;
+
+/**
+ * LJ CRC checksum utility
+ * 
+ * LJ message blocks are 80 bits in length as follows:
+ *    0 -  7: Bit reversals
+ *    8 - 15: Sync pattern
+ *   16 - 23: VRC
+ *   24 - 31: LRC
+ *   32 - 35: Function Code
+ *   36 - 63: Address
+ *   64 - 79: CRC
+ *   
+ * Each field is transmitted in big-endian bit order, including the CRC.  Only
+ * the Function and Address fields are protected by the CRC.
+ *        
+ * LJ uses a CRC-16 (0x6F63) with an initial fill of 0.
+ * 
+ * Generating Polynomial: x14 + x13 + x11 + x10 + x9 + x8 + x6 + x5 + x1 + 1 (0x6F63)
+ */
+public class CRCLJ
+{
+	private final static Logger mLog = LoggerFactory.getLogger( CRCLJ.class );
+	
+	private static final int MESSAGE_START = 32;
+	private static final int CRC_START = 64;
+
+	private static int[] CHECKSUMS = new int[] 
+	{
+		0x26EA, //Function 0 (MSB)
+		0x1375, //Function 1
+		0xBE0B, //Function 2
+		0xE8B4, //Function 3 (LSB)
+		0x745A, //Address 0 (MSB)
+		0x3A2D, //Address 1
+		0xAAA7, //Address 2
+		0xE2E2, //Address 3
+		0x7171, //Address 4
+		0x8F09, //Address 5
+		0xF035, //Address 6
+		0xCFAB, //Address 7
+		0xD064, //Address 8
+		0x6832, //Address 9
+		0x3419, //Address 10
+		0xADBD, //Address 11
+		0xE16F, //Address 12
+		0xC706, //Address 13
+		0x6383, //Address 14
+		0x8670, //Address 15
+		0x4338, //Address 16
+		0x219C, //Address 17
+		0x10CE, //Address 18
+		0x0867, //Address 19
+		0xB382, //Address 20
+		0x59C1, //Address 21
+		0x9B51, //Address 22
+		0xFA19, //Address 23
+		0xCABD, //Address 24
+		0xD2EF, //Address 25
+		0xDEC6, //Address 26
+		0x6F63, //Address 27
+		
+		//Single bit errors in the CRC Checksum
+		0x8000, //CRC 0
+		0x4000, //CRC 1
+		0x2000, //CRC 2
+		0x1000, //CRC 3
+		0x0800, //CRC 4
+		0x0400, //CRC 5
+		0x0200, //CRC 6
+		0x0100, //CRC 7
+		0x0080, //CRC 8
+		0x0040, //CRC 9
+		0x0020, //CRC 10
+		0x0010, //CRC 11
+		0x0008, //CRC 12
+		0x0004, //CRC 13
+		0x0002, //CRC 14
+		0x0001  //CRC 15
+	};
+
+	/**
+	 * Determines if message bits 16 - 47 pass the LJ CRC checksum 
+	 * contained in bits 48 - 63, using a lookup table of CRC checksum values
+	 * derived from the CRC-16 value
+	 */
+	public static CRC checkAndCorrect( BitSetBuffer msg )
+	{
+		int calculated = 0; //Starting value
+
+		/* Iterate the set bits and XOR running checksum with lookup value */
+		for (int i = msg.nextSetBit( MESSAGE_START ); 
+				 i >= MESSAGE_START && i < CRC_START; 
+				 i = msg.nextSetBit( i+1 ) ) 
+		{
+			calculated ^= CHECKSUMS[ i - MESSAGE_START ];
+		}
+
+		int checksum = getChecksum( msg );
+		
+		if( calculated == getChecksum( msg ) )
+		{
+			return CRC.PASSED;
+		}
+		else
+		{
+			int[] errors = findBitErrors( calculated ^ checksum );
+			
+			if( errors != null )
+			{
+				for( int error: errors )
+				{
+					msg.flip( MESSAGE_START + error );
+				}
+				
+				return CRC.CORRECTED;
+			}
+		}
+
+		return CRC.FAILED_CRC;
+	}
+
+	/**
+	 * Returns the integer value of the 15 bit crc checksum
+	 */
+    public static int getChecksum( BitSet msg )
+    {
+    	int retVal = 0;
+    	
+    	for( int x = 0; x < 16; x++ )
+    	{
+    		if( msg.get( x + CRC_START ) )
+    		{
+    			retVal += 1<<( 15 - x );
+    		}
+    	}
+    	
+    	return retVal;
+    }
+
+    /**
+     * Returns the bit position of any bit checksums that match the checksum
+     * error pattern.
+     */
+    public static int[] findBitErrors( int checksumError )
+    {
+    	/* One bit errors */
+		for( int x = 0; x < 48; x++ )
+		{
+			if( CHECKSUMS[ x ] == checksumError )
+			{
+				int[] errors = new int[ 1 ];
+
+				errors[ 0 ] = x;
+				
+				return errors;
+			}
+		}
+
+    	/* Two bit errors */
+		for( int x = 0; x < 48; x++ )
+		{
+			for( int y = 0; y < 48; y++ )
+			{
+				if( x != y && 
+					( CHECKSUMS[ x ] ^ CHECKSUMS[ y ] ) == checksumError )
+				{
+					int[] errors = new int[ 2 ];
+
+					errors[ 0 ] = x;
+					errors[ 1 ] = y;
+					
+					return errors;
+				}
+			}
+		}
+		
+		return null;
+    }
+}