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  • Author Author: oksbwn
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Arduino | Tutorial 3 | Serial Communication

oksbwn

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  • 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?

  • Former Member
    Former Member in reply to 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
          }
    }
  • 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...

  • 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

  • Former Member
    Former Member 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.

  • 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 is correct when he writes

     

    if (Serial) {
  // actions
}

     

    No magic. c++ and lots of digging into source.

  • Former Member
    Former Member in reply to Jan Cumps

    Ok,   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). image

     

    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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  • Former Member
    Former Member in reply to Jan Cumps

    Ok,   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). image

     

    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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