1

I'm programming an arduino uno with attached screen to be a clock and have the code below. It counts up from zero the milliseconds, seconds, minutes and hours and when it hits 24 hours it starts again from zero. Does anyone know how I can set the start time that it automatically starts at when it is reset? It will be a one off thing and I don't want it to interfere with the clock resetting to 0 hours after it hits 24. Thanks

#include <Wire.h>
#include <LiquidCrystal.h>

#define MILLIS_OVERFLOW 34359738

/**
 * Clock Variables
 */
unsigned long currentMillis, previousMillis, elapsedMillis;
int seconds, minutes, hours;

LiquidCrystal lcd(8, 9, 4, 5, 6, 7);

void setup()
{
    lcd.begin( 16, 2 ); 

}

void loop()
{
    setClock();
    /**
     * After set clock now you have 3 int variables with the current time
     */
     //seconds
     //minutes
     //hours
     lcd.setCursor ( 0, 1);
     lcd.print(hours);
     lcd.print(":");
     lcd.print(minutes);
     lcd.print(":");
     lcd.print(seconds);
     lcd.print(":");
     lcd.print(elapsedMillis);

}

void setClock()
{
    currentMillis = millis();
    /**
     * The only moment when currentMillis will be smaller than previousMillis
 * will be when millis() oveflows
 */
if (currentMillis < previousMillis){
    elapsedMillis += MILLIS_OVERFLOW - previousMillis + currentMillis;
} else {
    elapsedMillis += currentMillis - previousMillis;
}

/**
 * If we use equals 1000 its possible that because of the mentioned loop limitation
 * you check the difference when its value is (999) and on the next loop its value is (1001)
 */
if (elapsedMillis > 999){
    seconds++;
    elapsedMillis = elapsedMillis - 1000;
}

if (seconds == 60){
    minutes++;
    seconds = 0;
}
if (minutes == 60){
    hours++;
    minutes = 0;
}
if (hours == 24){
    hours = 0;
}

previousMillis = currentMillis;
}
3

Assume you want the clock to start at 08:42:21, change

int seconds, minutes, hours;

to

int seconds = 21, minutes = 42, hours = 8;

Note that you should always also initialize (as in the second case above), and not only declare (as in the first case) all variables before you first read their values. Otherwise, for local variables you cannot be sure what their initial value is. (Global variables are implicitly initialized to zero if not explicitly initialized.)

  • Your last sentence is incorrect. Global variables, like the ones discussed here, are implicitly initialized to zero if not explicitly initialized. The link you provide to support your assertion is for local variables. – Edgar Bonet Jun 1 '15 at 10:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.