Arduino | Tutorial 3 | Serial Communication

oksbwn

2 Jun 2016

Top Comments

Former Member over 9 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…
Former Member over 9 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…
Former Member over 9 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.

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Jan Cumps over 9 years ago
Are you sure that the code
```
if (Serial) {
  // communicate
}
```
makes sense in the video? Isn't Serial always different from 0, because it's a static object that has a non-zero address? even if Serial.begin(9600) fails?
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Former Member over 9 years ago in reply to Jan Cumps
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 hidden in the compilation/interpreter that treats Serial special. An if( Serial.isConnected() ) would be better. Secrets are bad engineering practices but what can you do? I cannot find any documentation of the static Serial object other than what's in the arduino.cc API. Have you?

To guard against a single spurious failure instance, I use this in setup() instead of the if-statement for Serial applications:
```
int count_down = 3; 
while( ! Serial ) {
      // Sleep 1 second if not yet ready.
      delay( 1000 );
      // Tried the maximum allowed times already?
      if( count_down-- < 1 ) {
           // Tried 3 seconds to get the serial connection ready - giving up.
           // Timeout error processing goes here
      }
}
```
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Jan Cumps over 9 years ago in reply to Former Member

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...
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Jan Cumps over 9 years ago in reply to Former Member
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
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Former Member over 9 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.
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Jan Cumps over 9 years ago in reply to Former Member
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.
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Comment

Jan Cumps over 9 years ago in reply to Former Member
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.
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Former Member over 9 years ago in reply to Jan Cumps

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 American Congress is useless so we might as well have a King. And, you'd probably do a lot better than the 2 individuals currently running President in any case!
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Jan Cumps over 9 years ago in reply to Former Member

When do the nominations officially close?
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Former Member over 9 years ago in reply to Jan Cumps

Also, as https://www.arduino.cc/en/Reference/Serial stated,

"The Arduino Leonardo board uses Serial1 to communicate via TTL (5V) serial on pins 0 (RX) and 1 (TX). Serial is reserved for USB CDC communication. For more information, refer to the Leonardo getting started page and hardware page."

I think that the same is true for all models.
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Former Member over 9 years ago in reply to Jan Cumps

Kings don't need a freakin' nomination. Just show up with your 3 dragons and legions of unsullied.
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Former Member over 9 years ago in reply to Jan Cumps

An interesting web page of details concerning Serial physical implementation and USB virtual serial devices (): https://www.arduino.cc/en/Guide/ArduinoLeonardoMicro
That page covers the Arduino/Genuino Uno too.
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