Arduino Cookbook, 3rd Edition

Using Time and Dates

12.0 Introduction

Managing time is a fundamental element of interactive computing. This chapter covers built-in Arduino functions and introduces many additional techniques for handling time delays, time measurement, and real-world times and dates. You’ll learn about Arduino’s built-in function for introducing delays into your sketch, as well as more advanced techniques for intermittently performing operations. Other recipes in this chapter cover how to measure time as it passes, and even how to use an external real-time clock for tracking time and dates.

12.1 Using millis to Determine Duration

Problem

You want to know how much time has elapsed since an event happened; for example, how long a switch has been held down.

Solution

The following sketch uses the millis() function to print how long a button was pressed (see Recipe 5.2 for details on how to connect the switch):

/*
  millisDuration sketch
  returns the number of milliseconds that a button has been pressed
 */

const int switchPin = 2;  // the number of the input pin

unsigned long startTime; // the value of millis when the switch is pressed
unsigned long duration;  // variable to store the duration

void setup()
{
  pinMode(switchPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop()
{
  if(digitalRead(switchPin) == LOW)
  {
    // here if the switch is pressed
    startTime = millis();
    while(digitalRead(switchPin) == LOW)
      ; // wait while the switch is still pressed
    unsigned long duration = millis() - startTime