How to utilize multiple software serial in libraries in Arduino? - gps

Question

I'm currently doing a project where I interface a GPS module, digital compass and a ZigBee (XBee modem) to Arduino. BTW, I'm using the Eclipse IDE to do this project, so I've broken down my code into several libraries.
XBee currently uses the hardware UART and it is working fine. The GPS module and the digital compass are both using Software Serial.
My problem is that if I instantiate both software serials, only the latest one will work.
Below is my sample code.
void CSensor::initSensors()
{
   //For GPS
   this->gpsSerial = new SoftwareSerial(2, 3);
   this->gpsSerial->begin(9600);
   //For digital compass
   this->compassSerial = new SoftwareSerial(6, 7);
   this->compassSerial->begin(9600);
}
gpsSerial is declared as a pointer member of CSensor class. -> SoftwareSerial* gpsSerial; compassSerial is also declared the same -> SoftwareSerial* compassSerial;
In the above sample code, only the digital compass will work, the GPS will not work. But if I interchange the positions of the code such that it will look like this:
void CSensor::initSensors()
{
   //For digital compass
   this->compassSerial = new SoftwareSerial(6, 7);
   this->compassSerial->begin(9600);
   //For GPS
   this->gpsSerial = new SoftwareSerial(2, 3);
   this->gpsSerial->begin(9600);
}
The GPS will now work, but the digital compass won't. I assume Software Serial has a problem. What is the solution?

Answer 1

One question: have you considered using an Arduino Mega for your project? With this board you get 4 (real) hardware UARTs, which should be enough for your purposes.
Using the "software serial" library is too tricky. It may work fine in some cases, but has important limitations when compared with a normal serial port, and it is very likely to provoke data loss.
When it comes to manage several serial devices, I would not recommend you to spend more time with "serial emulations". Just take a better microcontroller, and that is all.

Answer 2

According to the Ardiuno documentation:
The library has the following known limitations:
If using multiple software serial ports, only one can receive data at
a time.
So it looks like you'll have to add code to toggle between sampling from each software serial.
I know the GPS broadcasts at secondly intervals. So maybe after getting a valid GPS read, disconnect it, then watch the compass until the next scheduled GPS. A research question would be how long it takes to initialize things and keeping the code efficient enough.

Answer 3

Your SoftwareSerial is just fine. Like jdh said, Arduino can only read one software serial at a time. But if you are using the Sparkfun GPS, you can actually use the UART for GPS (by using the UART/Dline switch and changing the RX/TX pin numbers from 2, 3 to 0 & 1 respectively).
For this, keep the switch on dline and upload the code. After this change it back to UART sothat Arduino can read from GPS. This will work just fine.
For more details, check the UART section of the GPS tutorial.
http://www.sparkfun.com/tutorials/173

Answer 4

To set which SoftwareSerial port to listen on you use
mySerial.listen()
This means you have to wait for a message to arrive after setting listen(). Do this by checking mySerial.available()>0
The downside is that you miss messages from the other device so good program design becomes very important. But I am sure you know that :)

Answer 5

You should read a line of text from the GPS receiver, then use this->compassSerial->listen() to switch back to the compass, then back to the GPS. You'll lose some reports, but that may not be a constraint for you.