# HG changeset patch
# User mkanning@CL-ENS241-10.cedarville.edu
# Date 2013-01-10 15:03:18
# Node ID e5c11279c2a031ff0b6570d8ccca42b3f8d6fbf2
# Parent  1614c3d5e9cd1ed1415177d2b71397fb3de20b87

checksum calculator and comparator for NMEA

diff --git a/master/master/lib/trackuinoGPS/gpsMKa.c b/master/master/lib/trackuinoGPS/gpsMKa.c
--- a/master/master/lib/trackuinoGPS/gpsMKa.c
+++ b/master/master/lib/trackuinoGPS/gpsMKa.c
@@ -42,13 +42,14 @@ uint8_t course[5];		//xxx.x
 uint8_t datestamp[6];	//ddmmyy
 uint8_t variation[6];	//xxx.xb
 int calculatedChecksum;
+int receivedChecksum;
 
 // transmission state machine
 enum decodeState {
 	//shared fields
 	INITIALIZE=0,
 	GET_TYPE,
-	GPS_CHECKSUM,	//need to find out how this is calculated/compared
+	GPS_CHECKSUM,	//XOR of all the bytes between the $ and the * (not including the delimiters themselves), written in hexadecimal
 	//GGA data fields
 	GGA_TIME,
 	GGA_LATITUDE,
@@ -410,7 +411,7 @@ void parse_gps_transmission(void){
 	//magnetic variation
 	else if(decodeState == RMC_MAG_VARIATION)
 	{
-		if (byte == ',') //end of this data type
+		if (byte == '*') //end of this data type
 		{
 			decodeState = GPS_CHECKSUM;
 			numBytes = 0; //prep for next phase of parse
@@ -429,6 +430,18 @@ void parse_gps_transmission(void){
 	{
 		if (numBytes == 2) //end of data - terminating character ??
 		{
+			//checksum calculator for testing http://www.hhhh.org/wiml/proj/nmeaxor.html
+			//TODO: must determine what to do with correct and incorrect messages
+			receivedChecksum = checksum[0] + (checksum[1]*16);	//convert bytes to int
+			if(calculatedChecksum==receivedChecksum)
+			{
+				
+			}
+			else
+			{
+				
+			}
+			
 			decodeState = INITIALIZE;
 			numBytes = 0; //prep for next phase of parse
 		}
@@ -439,8 +452,13 @@ void parse_gps_transmission(void){
 		}
 	}
 	
-	//input byte into running checksum
-	XORbyteWithChecksum(byte);
+	if (decodeState!=GPS_CHECKSUM && decodeState!=INITIALIZE)	//want bytes between '$' and '*'
+	{
+		//input byte into running checksum
+		XORbyteWithChecksum(byte);
+	}
+	
+
 }
 /// MKa GPS transmission parser END