Arduino Uno lag in simple loop

Question

I have a simple Arduino sketch which I have been playing with on my Uno.

The idea is simple, the user turns a potentiate (pin A2), when they pass a certain threshold a LED is turned on (pin 7).

Here is the code:

// Digital pins
const byte ledPin = 7;
// Analog pins
const byte userInputPotPin = A2;
// Potentiometer constants
const int userInputPotMinimumValue = 0;
const int userInputPotMaximumValue = 1023;
const int userInputMinimumValue = 1;
const int userInputMaximumValue = 100;
// Variables
const int threshold = 90;
void setup()
{
 Serial.begin(9600);
 // Digital pins
 pinMode(ledPin, OUTPUT);
 // Analog pins
 pinMode(userInputPotPin, INPUT);
}
void loop()
{
 int rawValue = analogRead(userInputPotPin);
 int mappedValue = map(rawValue, userInputPotMinimumValue, userInputPotMaximumValue, userInputMinimumValue, userInputMaximumValue);
 String messageText;
 messageText += "Raw: " + String(rawValue) + "\t";
 messageText += "Mapped:" + String(mappedValue);
 if(mappedValue > threshold)
 {
 messageText += "\tThreshold exceeded!";
 digitalWrite(ledPin, HIGH);
 }
 else
 {
 digitalWrite(ledPin, LOW);
 }
 Serial.println(messageText);
 delay(100);
}

The problem is that when it is run the hardware is very unresponsive - I can turn the pot, but I don't see the light change for several minutes. The log messages in the serial monitor also seem to be lagging behind.

I have tried:
* Removing the Serial statements (so there is no serial communication when the sketch is running), and just observing the LED. This makes no difference.
* Increasing the value in the delay up to 1000. This makes no difference in the responsiveness.

However, when I run the sketch on my Mega2560 the problem goes away and the circuit is as responsive as I expect.

Is there something wrong with my Uno board?

• ocrdu: What happens when you also comment out all the code that uses and contructs the String messageText?
• Kwasmich: Your MCU is busy printing text and waiting.
• Edgar Bonet: I can't see anything in your code that could make it so slow. Could you try with just void loop(){Serial.println(analogRead(A2));delay(100);}?

Answer 1

It turns out the problem was the A2 pin on the board.

When I used the A0 pin (or a different board) the problem goes away.

I realised something was wrong when I noticed the values from the pot were strange (I expected values in the range 0 - 1023, but was receiving values from 170-250 ish).

• Michel Keijzers: Btw, you can use the Arduino 'map' command to map a range to another range.

Answer 2

The String+ operator is very expensive. High likely what it does is:

• Create a new block of memory to accommodate the old string content and the to-be-concatenated text.
• Copy the current String to a new string location
• Concatenate the string
• Free the old memory

Besides creating memory gaps which can be disastrous for the just 2 KB SRAM of the Arduino Uno, it takes a lot of time.

Some tips:

• Do not use Strings
• Do not concatenate strings if not needed
• To save more time, use short strings instead

If you really need to debug a time critical part, copy numbers you want to print to an array, and AFTER you do all 'real' code, start printing the numbers.

• chrisl: Also there is no reason to concat strings for serial output. Just print the parts one after another. To get "Number: 5 bananas" you could use Serial.print("Number: "); Serial.print(number_variable); Serial.println(" bananas")
• Michel Keijzers: @chrisl that's indeed my second tip.
• Hoppy: Thanks for the excellent advice. Unfortunately I can't mark it as the answer to the question as my issue was caused by something else (see my answer).
• Michel Keijzers: No problem, btw, you can accept your own answer too.