GPS Breakout Example Program Problem, sort of in a rush...

Hello all!

Recently, I was trying to get a Adafruit Ultimate GPS Breakout to work using an example program from the GitHub Library. I installed the library correctly, I’ve done it before, and I can get the program open, but after uploading it nothing happens. There is some modification I’ve had to do, as written in the comments, but that was with Serial1 ports and such. The program was written by Adafruit, and I got it to display some stuff on the Serial Monitor, but it wasn’t able to get a fix, even when it was outside.

I left it outside for an hour, before realizing that I could get correct data from it without the program- if you open up the Serial Monitor without any code and the wire RX/TX directly plugged in the TX0/RX0, it will constantly update info. However, when I try to parse it, it does not show anything anymore, just the first sentence: “Adafruit GPS library basic test!”

Why is this happening, and how do I fix it? I am using an Arduino Mega R3 with the power wires plugged in correctly, RX into RX0, and TX into TX0.

Also, I have never seen it do anything but flash a red LED once every 15 sec. Is it supposed to do only that?


// Test code for Adafruit GPS modules using MTK3329/MTK3339 driver
// This code shows how to listen to the GPS module in an interrupt
// which allows the program to have more 'freedom' - just parse
// when a new NMEA sentence is available! Then access data when
// desired.
// Tested and works great with the Adafruit Ultimate GPS module
// using MTK33x9 chipset
//    ------>
// Pick one up today at the Adafruit electronics shop 
// and help support open source hardware & software! -ada

#include <Adafruit_GPS.h>
#include <SoftwareSerial.h>

// If you're using a GPS module:
// Connect the GPS Power pin to 5V
// Connect the GPS Ground pin to ground
// If using software serial (sketch example default):
//   Connect the GPS TX (transmit) pin to Digital 3
//   Connect the GPS RX (receive) pin to Digital 2
// If using hardware serial (e.g. Arduino Mega):
//   Connect the GPS TX (transmit) pin to Arduino RX1, RX2 or RX3
//   Connect the GPS RX (receive) pin to matching TX1, TX2 or TX3

// If you're using the Adafruit GPS shield, change 
// SoftwareSerial mySerial(3, 2); -> SoftwareSerial mySerial(8, 7);
// and make sure the switch is set to SoftSerial

// If using software serial, keep this line enabled
// (you can change the pin numbers to match your wiring):
//SoftwareSerial mySerial(8, 7);

// If using hardware serial (e.g. Arduino Mega), comment out the
// above SoftwareSerial line, and enable this line instead
// (you can change the Serial number to match your wiring):

HardwareSerial mySerial = Serial1;

Adafruit_GPS GPS(&mySerial);

// Set GPSECHO to 'false' to turn off echoing the GPS data to the Serial console
// Set to 'true' if you want to debug and listen to the raw GPS sentences. 
#define GPSECHO  true

// 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

void setup()  
  // connect at 115200 so we can read the GPS fast enough and echo without dropping chars
  // also spit it out
  Serial.println("Adafruit GPS library basic test!");

  // 9600 NMEA is the default baud rate for Adafruit MTK GPS's- some use 4800
  // uncomment this line to turn on RMC (recommended minimum) and GGA (fix data) including altitude
  // uncomment this line to turn on only the "minimum recommended" data
  // For parsing data, we don't suggest using anything but either RMC only or RMC+GGA since
  // the parser doesn't care about other sentences at this time
  // Set the update rate
  GPS.sendCommand(PMTK_SET_NMEA_UPDATE_1HZ);   // 1 Hz update rate
  // For the parsing code to work nicely and have time to sort thru the data, and
  // print it out we don't suggest using anything higher than 1 Hz

  // Request updates on antenna status, comment out to keep quiet

  // 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!

  // Ask for firmware version

// Interrupt is called once a millisecond, looks for any new GPS data, and stores it
  char c =;
  // 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. 

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;

uint32_t timer = millis();
void loop()                     // run over and over again
  // in case you are not using the interrupt above, you'll
  // need to 'hand query' the GPS, not suggested :(
  if (! usingInterrupt) {
    // read data from the GPS in the 'main loop'
    char c =;
    // if you want to debug, this is a good time to do it!
    if (GPSECHO)
      if (c) Serial.print(c);
  // if a sentence is received, we can check the checksum, parse it...
  if (GPS.newNMEAreceived()) {
    // a tricky thing here is if we print the NMEA sentence, or data
    // we end up not listening and catching other sentences! 
    // so be very wary if using OUTPUT_ALLDATA and trytng to print out data
    //Serial.println(GPS.lastNMEA());   // this also sets the newNMEAreceived() flag to false
    if (!GPS.parse(GPS.lastNMEA()))   // this also sets the newNMEAreceived() flag to false
      return;  // we can fail to parse a sentence in which case we should just wait for another

  // if millis() or timer wraps around, we'll just reset it
  if (timer > millis())  timer = millis();

  // approximately every 2 seconds or so, print out the current stats
  if (millis() - timer > 2000) { 
    timer = millis(); // reset the timer
    Serial.print("\nTime: ");
    Serial.print(GPS.hour, DEC); Serial.print(':');
    Serial.print(GPS.minute, DEC); Serial.print(':');
    Serial.print(GPS.seconds, DEC); Serial.print('.');
    Serial.print("Date: ");
    Serial.print(, DEC); Serial.print('/');
    Serial.print(GPS.month, DEC); Serial.print("/20");
    Serial.println(GPS.year, DEC);
    Serial.print("Fix: "); Serial.print((int)GPS.fix);
    Serial.print(" quality: "); Serial.println((int)GPS.fixquality); 
    if (GPS.fix) {
      Serial.print("Location: ");
      Serial.print(GPS.latitude, 4); Serial.print(;
      Serial.print(", "); 
      Serial.print(GPS.longitude, 4); Serial.println(GPS.lon);
      Serial.print("Location (in degrees, works with Google Maps): ");
      Serial.print(GPS.latitudeDegrees, 4);
      Serial.print(", "); 
      Serial.println(GPS.longitudeDegrees, 4);
      Serial.print("Speed (knots): "); Serial.println(GPS.speed);
      Serial.print("Angle: "); Serial.println(GPS.angle);
      Serial.print("Altitude: "); Serial.println(GPS.altitude);
      Serial.print("Satellites: "); Serial.println((int)GPS.satellites);

RX into RX0, and TX into TX0.

If RX and TX are on the GPS and RX0 and TX0 are on the Arduino, this will work better if RX goes to TX0 and TX goes to RX0.

I have never seen it do anything but flash a red LED once every 15 sec

That is a good sign. It means that the GPS has found and locked onto the satellites as explained on the Adafruit website.

When is this assignment due?


Ok, is the RX to TX thing b/c one transmits and the other receives? If it is, well, uhh, whoops...

How do I modify the program accordingly? When I uploaded it, I got the correct stuff, but it just says "GPS fix: 0" (meaning it thinks it's not fixed, even when the LED is blinking correctly). I have a feeling that's not because I'm inside but because its problem is in the code.

As for the LED, duh again. Thanks for telling me! :slight_smile:

This assignment is due in two weeks, and I have most everything but programs set up.

I'm also having trouble with an HMC5883l compass... It's the same problem I had with the GPS- one line of Serial Monitor Code and nothing else popping up. It doesn't use TX/RX, though - it uses I2C SDA+SCL pins. Any ideas, since it's the same obstacle?

I am using an Arduino Mega R3 with the power wires plugged in correctly, RX into RX0, and TX into TX0

But the code itself says:

// If using hardware serial (e.g. Arduino Mega):
//   Connect the GPS TX (transmit) pin to Arduino RX1, RX2 or RX3
//   Connect the GPS RX (receive) pin to matching TX1, TX2 or TX3

And the code is using Serial1;

HardwareSerial mySerial = Serial1;

so you'd better use RX1 and TX1.

It really pays to read the documentation.


Yeah, that's what I thought. I'll do that now. Thanks!

Ok, I changed the wiring around, and I got all good lights, but the Serial Monitor won't open up! I've re uploaded, tried accessing it in the different menus, keyboard shortcut, all that, and nope! It's disappeared! What do I do now?