Added interrupt support

Added interrupt support to shield_sdlog sketch

examples/shield_sdlog/shield_sdlog.ino

@@ -26,6 +26,11 @@ Adafruit_GPS GPS(&mySerial);
 /* set to true to only log to SD when GPS has a fix, for debugging, keep it false */
 #define LOG_FIXONLY false  
 
+// this keeps track of whether we're using the interrupt
+// off by default!
+boolean usingInterrupt = false;
+void useInterrupt(boolean); // Func prototype keeps Arduino 0023 happy
+
 // Set the pins used
 #define chipSelect 10
 #define ledPin 13
@@ -46,7 +51,7 @@ uint8_t parseHex(char c) {
 
 // blink out an error code
 void error(uint8_t errno) {
-/*
+  /*
   if (SD.errorCode()) {
    putstring("SD error: ");
    Serial.print(card.errorCode(), HEX);
@@ -102,10 +107,12 @@ void setup() {
 
   logfile = SD.open(filename, FILE_WRITE);
   if( ! logfile ) {
-    Serial.print("Couldnt create "); Serial.println(filename);
+    Serial.print("Couldnt create "); 
+    Serial.println(filename);
     error(3);
   }
-  Serial.print("Writing to "); Serial.println(filename);
+  Serial.print("Writing to "); 
+  Serial.println(filename);
 
   // connect to the GPS at the desired rate
   GPS.begin(9600);
@@ -122,13 +129,54 @@ void setup() {
   // Turn off updates on antenna status, if the firmware permits it
   GPS.sendCommand(PGCMD_NOANTENNA);
 
+  // the nice thing about this code is you can have a timer0 interrupt go off
+  // every 1 millisecond, and read data from the GPS for you. that makes the
+  // loop code a heck of a lot easier!
+  useInterrupt(true);
+
   Serial.println("Ready!");
 }
 
+
+// Interrupt is called once a millisecond, looks for any new GPS data, and stores it
+SIGNAL(TIMER0_COMPA_vect) {
+  char c;
+  while (mySerial.available())
+  {
+    c = GPS.read();
+    // if you want to debug, this is a good time to do it!
+#ifdef UDR0
+    if (GPSECHO)
+      if (c) UDR0 = c;  
+    // writing direct to UDR0 is much much faster than Serial.print 
+    // but only one character can be written at a time. 
+#endif
+  }
+}
+
+void useInterrupt(boolean v) {
+  if (v) {
+    // Timer0 is already used for millis() - we'll just interrupt somewhere
+    // in the middle and call the "Compare A" function above
+    OCR0A = 0xAF;
+    TIMSK0 |= _BV(OCIE0A);
+    usingInterrupt = true;
+  } 
+  else {
+    // do not call the interrupt function COMPA anymore
+    TIMSK0 &= ~_BV(OCIE0A);
+    usingInterrupt = false;
+  }
+}
+
 void loop() {
-  char c = GPS.read();
+  char c;
+  while (mySerial.available())
+  {
+    c = GPS.read();
     if (GPSECHO)
       if (c)   Serial.print(c);
+  }
   
   // if a sentence is received, we can check the checksum, parse it...
   if (GPS.newNMEAreceived()) {
@@ -161,3 +209,4 @@ void loop() {
 
 
 /* End code */
+