2 GPS Devices Connected to Mega 2560

I working to try to read two GPS units (Adafuit GPS Breakout) using a single mega 2560 board. I was able to successfully parse the GPS data from a single unit with this code:

// 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
//    ------> http://www.adafruit.com/products/746
// 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(3, 2);

// 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.begin(115200);
  Serial.println("Adafruit GPS library basic test!");

  // 9600 NMEA is the default baud rate for Adafruit MTK GPS's- some use 4800
  GPS.begin(9600);
  
  // uncomment this line to turn on RMC (recommended minimum) and GGA (fix data) including altitude
  GPS.sendCommand(PMTK_SET_NMEA_OUTPUT_RMCGGA);
  // uncomment this line to turn on only the "minimum recommended" data
  //GPS.sendCommand(PMTK_SET_NMEA_OUTPUT_RMCONLY);
  // 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
  GPS.sendCommand(PGCMD_ANTENNA);

  // 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);

  delay(1000);
  // Ask for firmware version
  mySerial.println(PMTK_Q_RELEASE);
}


// Interrupt is called once a millisecond, looks for any new GPS data, and stores it
SIGNAL(TIMER0_COMPA_vect) {
  char 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;
  }
}

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 = GPS.read();
    // 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.println(GPS.milliseconds);
    Serial.print("Date: ");
    Serial.print(GPS.day, 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(GPS.lat);
      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);
    }
  }
}

Then I “attempted” to extend it to parse two units, which was giving me some weird issues with random numbers being inserted into the serial monitor output.

I’m not sure if I should be using the software serial option or the UART (hardware serial). I’m also not sure if I should try the TinyGPS+ library over the adafruit gps library.

Any advice would be nice.

I regret to show you my butchered code for my attempt to parse two GPS outputs using the hardware serial.

// Test code for Adafruit GPS modules using MTK3329/MTK3339 driver
//

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


HardwareSerial mySerial_1 = Serial1;
HardwareSerial mySerial_2 = Serial2;


Adafruit_GPS GPS_1(&mySerial_1);
Adafruit_GPS GPS_2(&mySerial_2);


// 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.begin(115200);
  Serial.println("Adafruit GPS library basic test!");

  // 9600 NMEA is the default baud rate for Adafruit MTK GPS's- some use 4800
  GPS_1.begin(9600);
  
  // uncomment this line to turn on RMC (recommended minimum) and GGA (fix data) including altitude
  GPS_1.sendCommand(PMTK_SET_NMEA_OUTPUT_RMCGGA);
  // uncomment this line to turn on only the "minimum recommended" data
  //GPS_1.sendCommand(PMTK_SET_NMEA_OUTPUT_RMCONLY);
  // 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_1.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
  GPS_1.sendCommand(PGCMD_ANTENNA);

  // 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);

  delay(1000);
  // Ask for firmware version
  mySerial_1.println(PMTK_Q_RELEASE);
  
  // ************ ADDED GPS2******************
  GPS_2.begin(9600);
  GPS_2.sendCommand(PMTK_SET_NMEA_OUTPUT_RMCGGA);
  GPS_2.sendCommand(PMTK_SET_NMEA_UPDATE_1HZ);
  GPS_2.sendCommand(PGCMD_ANTENNA);
  useInterrupt(true);
  delay(1000);
  // Ask for firmware version
  mySerial_2.println(PMTK_Q_RELEASE);
}


// Interrupt is called once a millisecond, looks for any new GPS data, and stores it
SIGNAL(TIMER0_COMPA_vect) {
  char c1 = GPS_1.read();
  // if you want to debug, this is a good time to do it!
#ifdef UDR0
  if (GPSECHO)
    if (c1) UDR0 = c1;  
    // writing direct to UDR0 is much much faster than Serial.print 
    // but only one character can be written at a time. 
#endif
  char c2 = GPS_2.read();
  // if you want to debug, this is a good time to do it!
#ifdef UDR0
  if (GPSECHO)
    if (c2) UDR0 = c2;  
    // 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;
  }
}

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 c1 = GPS_1.read();
    // if you want to debug, this is a good time to do it!
    if (GPSECHO)
      if (c1) Serial.print(c1);
    char c2 = GPS_2.read();
    // if you want to debug, this is a good time to do it!
    if (GPSECHO)
      if (c2) Serial.print(c2);
  }
  
  // if a sentence is received, we can check the checksum, parse it...
  if (GPS_1.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_1.lastNMEA());   // this also sets the newNMEAreceived() flag to false
  
    if (!GPS_1.parse(GPS_1.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 (GPS_2.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_1.lastNMEA());   // this also sets the newNMEAreceived() flag to false
  
    if (!GPS_2.parse(GPS_2.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 1: ");
    Serial.print(GPS_1.hour, DEC); Serial.print(':');
    Serial.print(GPS_1.minute, DEC); Serial.print(':');
    Serial.print(GPS_1.seconds, DEC); Serial.print('.');
    Serial.println(GPS_1.milliseconds);
    Serial.print("Date: ");
    Serial.print(GPS_1.day, DEC); Serial.print('/');
    Serial.print(GPS_1.month, DEC); Serial.print("/20");
    Serial.println(GPS_1.year, DEC);
    Serial.print("Fix: "); Serial.print((int)GPS_1.fix);
    Serial.print(" quality: "); Serial.println((int)GPS_1.fixquality); 
    if (GPS_1.fix) {
      Serial.print("Location 1: ");
      Serial.print(GPS_1.latitude, 4); Serial.print(GPS_1.lat);
      Serial.print(", "); 
      Serial.print(GPS_1.longitude, 4); Serial.println(GPS_1.lon);
      Serial.print("Location (in degrees, works with Google Maps): ");
      Serial.print(GPS_1.latitudeDegrees, 4);
      Serial.print(", "); 
      Serial.println(GPS_1.longitudeDegrees, 4);
      
      Serial.print("Speed (knots): "); Serial.println(GPS_1.speed);
      Serial.print("Angle: "); Serial.println(GPS_1.angle);
      Serial.print("Altitude: "); Serial.println(GPS_1.altitude);
      Serial.print("Satellites: "); Serial.println((int)GPS_1.satellites);
    }
    Serial.print("\n*************** GPS_2********* ");
    Serial.print("\nTime 2: ");
    Serial.print(GPS_2.hour, DEC); Serial.print(':');
    Serial.print(GPS_2.minute, DEC); Serial.print(':');
    Serial.print(GPS_2.seconds, DEC); Serial.print('.');
    Serial.println(GPS_2.milliseconds);
    Serial.print("Date: ");
    Serial.print(GPS_2.day, DEC); Serial.print('/');
    Serial.print(GPS_2.month, DEC); Serial.print("/20");
    Serial.println(GPS_2.year, DEC);
    Serial.print("Fix: "); Serial.print((int)GPS_2.fix);
    Serial.print(" quality: "); Serial.println((int)GPS_2.fixquality); 
    if (GPS_2.fix) {
      Serial.print("Location 2: ");
      Serial.print(GPS_2.latitude, 4); Serial.print(GPS_2.lat);
      Serial.print(", "); 
      Serial.print(GPS_2.longitude, 4); Serial.println(GPS_2.lon);
      Serial.print("Location (in degrees, works with Google Maps): ");
      Serial.print(GPS_2.latitudeDegrees, 4);
      Serial.print(", "); 
      Serial.println(GPS_2.longitudeDegrees, 4);
      
      Serial.print("Speed (knots): "); Serial.println(GPS_2.speed);
      Serial.print("Angle: "); Serial.println(GPS_2.angle);
      Serial.print("Altitude: "); Serial.println(GPS_2.altitude);
      Serial.print("Satellites: "); Serial.println((int)GPS_2.satellites);
    }
  }
}

I working to try to read two GPS units

May I ask why? Even after stabilization, the RMS value of the GPS calculations may not fully agree which introduces an extended error. What I am saying is the RMS fluctuations for these time pulse is 30 nS for the Ublox-Neo6. Having 2 independent units does not minimize this error.

I am very familiar with the Arduino and the Adafruit GPS library. I have hacked it for my GPS Color Clock. Essentially, I am just using the RMC sentence for time+date.

Limor uses a dual-buffer arrangement in her code. I have not looked at TinyGPS+ but you should look into using the available-memory code (Playground) to really determine what 2 instances will leave for stack, etc. If you do not understand Arduino memory, this should Help!

GPS modules generally update 1 time per second (default). Some more expensive units 4 times a second and some more expensive ones 10 times a second. There is to my knowledge no way to force the two separate GPS modules into lock-step sync… So the sentences are utilizing 2 buffers and parsing is only started on a CR/LF received.

In looking at your code, I do not understand the need for 2 GPS units.

Ray

I'd like to test the accuracy between two units. I'm going to try to use one gps to located the another gps unit that is connected to the same computer. I bought two external antennas with 10m extensions to see how close I could get to locate the other. I'm thinking I might just need to connect two arduinos to one computer.

TFinleyosu: I'd like to test the accuracy between two units. I'm going to try to use one gps to located the another gps unit that is connected to the same computer. I bought two external antennas with 10m extensions to see how close I could get to locate the other. I'm thinking I might just need to connect two arduinos to one computer.

This is the kind of whacky thing I'd do just to pass some time. :)

Another thing I've seen is a GPS used as a mouse input. Then if you attach it to a laptop and drive around a deserted car park, Open paint on the laptop, you can draw a face.

This code isn’t needed:

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

The unsigned long math works out fine when you do Current-Past compared to Interval.

I don’t think it is advisable to call the GPS library functions from an interrupt. They should be called from loop().