Arduino | Tutorial 3 | Serial Communication

oksbwn

2 Jun 2016

Top Comments

e14 Contributor over 10 years ago in reply to Jan Cumps +1

Jan Cumps Apparently, a reference to Serial in an if or while argument context evaluates to true if the specified serial port is available else there is a permanent/temporary error. There must be something…
e14 Contributor over 10 years ago in reply to Jan Cumps +1

Ok, Jan Cumps earned the "King for the Day" title. I've been busy cleaning up my own mess (one of those "no good deed goes unpunished" stories). King of which country? How about the USA? Stay longer. The…
e14 Contributor over 10 years ago in reply to Jan Cumps +1

Kings don't need a freakin' nomination. Just show up with your 3 dragons and legions of unsullied.

Jan Cumps over 10 years ago in reply to e14 Contributor
I looked somewhat further:

(this is true for boards where the value USBCON is defined, it doesn't work if HardwareSerial is used - they always return true. For those boards the object is created in source file https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/HardwareSerial0.cpp - in the end it's the board that defines if you're using HardwareSerial or Serial_)

the object named Serial is declared in CDC.cpp
https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/CDC.cpp
```
Serial_ Serial;
```
You can see that the class is Serial_ not HardwareSerial

And that class has the bool operator overloaded in a proper way
```
Serial_::operator bool() {
  bool result = false;
  if (_usbLineInfo.lineState > 0) 
    result = true;
  delay(10);
  return result;
}
```
And that means that
```
if (Serial) 
```
returns true if the line state is active, and false if the line hasn't been established.
So oksbwn is correct when he writes
```
if (Serial) {
  // actions
}
```
No magic. c++ and lots of digging into source.
- Cancel
- Vote Up 0 Vote Down
- Sign in to reply
- More
- Cancel
e14 Contributor over 10 years ago in reply to Jan Cumps

See the SerialEvent example at https://www.arduino.cc/en/Tutorial/SerialEvent
Note that there are no include statements either for HardwareSerial.h nor SoftwareSerial.h. In fact, there are no include statements at all.
So, how is it that this "magical" Serial object is accounted for?
There is not even an instantiation nor a constructor invoked by the user.

It still looks to me that the Arduino IDE is adding something to the sketch before submitting it for C++ compilation.
- Cancel
- Vote Up 0 Vote Down
- Sign in to reply
- More
- Cancel
Jan Cumps over 10 years ago in reply to e14 Contributor
I have been digging in the source code.

The files are available on github

The one we're talking about is:
https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/HardwareSerial.h

Relevant pieces of code
```
class HardwareSerial : public Stream
{
// ...
  public:
    operator bool() { return true; }
// ...
}

// ... 
  extern HardwareSerial Serial;
```
So it looks like the bool operator is indeed overridden for the class HardwareSerial (also checked SoftwareSerial, it's the same mechanism there), but it always returns true, whatever the state of the connection
- Cancel
- Vote Up 0 Vote Down
- Sign in to reply
- More
- Cancel
Jan Cumps over 10 years ago in reply to e14 Contributor

The compiler is a standard c++ compiler and the linker is also a known one. So whatever the Serial library does must be a c++ construct.

The source for the System class is open, so we can have a look ourselves. Let's do that.
The first who finds it is king for a day...
- Cancel
- Vote Up 0 Vote Down
- Sign in to reply
- More
- Cancel
oksbwn over 10 years ago in reply to Jan Cumps

Jan Cumps : @richard_elkins is right. Actually I have only demonstrated how to use that. It's up to the user to put logic whatever he wants like he might reboot or he might stop execution or wait for certain time etc.
- Cancel
- Vote Up 0 Vote Down
- Sign in to reply
- More
- Cancel