To keep track of time I used only the arduino, and the standard arduino function millis(). This method won't be completely exact, because the 16Mhz crystal oscillator, that decides the clock frequency of the arduinos CPU, probably won't be exactly 16 000 000 Hz. The good news is that you can measure how inexact your clock is and compensate for it in your code, because the offset should be constant for a given period of time.Measure how inexact your arduino is:
As said earlier, the arduino will have a small time error, this error depends on the crystal oscillator and will be different for every arduino, to measure how much my arduino clock differed from the correct time, I programmed it to print out the time ( the hour, minute and second variables) via serial. I then let it run for a long time (Over night and more) and compared the arduino time with a clock I knew was exact, at the start and the end of my measuring period. Divide the time difference with the time the test took to calculate the error each hour. I found that my arduino is about 0.4 seconds to fast every hour. I used exacttimenow.com to get exact time, but if you have a watch you know is very exact, feel free to use that instead.
The code I used to keep the time is an adaptation of some code I found on the arduino forums
. I rewrote it with if-statements instead of division and modulo, to see if there would be any speed difference and found that the if-version is more than 15 times faster (although both are still quite fast, more info about the test here
Since I want the other stuff in my main loop (like checking the touch sensor, checking for button presses, etc.) to happen as often as possible, I used the faster version.
Every time the clock function is called it adds the time in milliseconds since last it was called to a variable m, when one second has passed (m>999) the second variable is increased by one and m is reset. When the seconds variable reaches 60, the minute variable will be increased by one, and seconds will be set to zero. The same thing happens with the minutes variable; when it reaches 60, add 1 to hours and reset minutes. The hour variable will be reset when it reaches 24.
To compensate that my arduino is 0.4 seconds faster evey hour, I decrease the seconds with two seconds every fifth hour.
The clock() function:
// CLOCK VARIABLES:
#define MAX_MILLIS_VALUE 34359738
unsigned long current_millis_value = 0;
unsigned long previous_millis_value = 0;
unsigned long m = 0;
int seconds = 0;
int minutes = 0;
int hours = 0;
current_millis_value = millis();
if (current_millis_value < previous_millis_value) // if millis overflows
m += MAX_MILLIS_VALUE - previous_millis_value + current_millis_value;
else // if millis has not overflown
m += current_millis_value - previous_millis_value;
m = m-1000;
if (seconds>59) // if seconds == 60
seconds = 0;
if (minutes>59) // if minutes == 60
minutes = 0;
if(hours%5==0) // adjust the time with -2 seconds every 5th hour.
seconds = seconds - 2; // this will cause seconds to be -2,
// therefore seconds can't be unsigned.
if (hours>23) // if hours == 24
hours = 0;
previous_millis_value = current_millis_value;