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Serial debug output looking messy

I've got this quite basic code running on an Arduino Mega 2560, and the output is looking like I have the serial settings wrong, though I can't work out what to change.

void setup() 
{
 Serial.begin(9600);
 Serial.println("test123");
}
void loop()
{
 Serial.println("test" + millis());
}

The output looks like this:

×ばつ¤¿ïßûÏûýoÏ¿îãõI
õfwi÷`p¬1⁄2Ïa»QáÀa»¦·ð¦·Z©÷
test123
D
4ñjü ̄÷~μÞMz1Ý~1⁄4
Ûþ}iû}ÒÖ¦Ç¿÷ÿÂï1⁄2Õ3⁄4;îö

I'm using the inbuilt "Serial Monitor" in the Arduino suite with settings 9600 baud and no line endings.

I see that the first println in setup() is sending the data as expected but all other comms are coming through as nonsense.

Any suggestions of what to do?

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Great, thank you. Can you also explain why string concatenation per arduino.cc/en/Tutorial/StringAdditionOperator isn't working how I expected it would in this instance, when one of their examples on the page is specifically stringThree = stringOne + millis();?

bdx
– bdx

2014年12月25日 10:38:46 +00:00
Commented Dec 25, 2014 at 10:38
2

@bdx You can concatenate String objects like that because they overload the plus (+) operator. String literals in C/C++ (anything in double quotation marks "like this") work differently. You can read more about the difference here: arduino.cc/en/Reference/String

Peter Bloomfield
– Peter Bloomfield

2014年12月25日 17:33:55 +00:00
Commented Dec 25, 2014 at 17:33
I see. My assumption that a string literal would create a String object was false, it creates a null-terminated character array, and the String type also has an overload on the = operator to coerce a char[] to a String.

bdx
– bdx

2014年12月25日 20:34:17 +00:00
Commented Dec 25, 2014 at 20:34
You can explicitly construct a String object by doing this: String("test"). It uses more memory than a string literal alone though, so it's important to be careful how you use it.

Peter Bloomfield
– Peter Bloomfield

2014年12月25日 22:49:50 +00:00
Commented Dec 25, 2014 at 22:49

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