HH:mm clock overflowing

Projects Programming
1

Hello, I'm building a timer for something, it has an input of seconds and displays time in hh:mm.

For some reason the time goes wrong at the 10hr mark and I'm really confused as to why, any help would be appreciated.

long sec = 35999;

  int Hour = sec / 3600;
  Serial.print("Hour: "); Serial.println(Hour);
  unsigned long val = Hour * 3600;
  Serial.print("val: "); Serial.println(val);
  long Minutes = sec - val;
  Serial.print("Minutes: "); Serial.println(Minutes);
  Minutes /= 60;
  Serial.print("Minutes: "); Serial.println(Minutes);
  int minLsd = Minutes % 10;
  Serial.print("minLsd: "); Serial.println(minLsd);
  int minMsd = Minutes / 10;
  Serial.print("minMsd: "); Serial.println(minMsd);

  Serial.print(Hour); Serial.print("h"); Serial.print(minMsd); Serial.println(minLsd);   

  sec = 36000;

  Hour = sec / 3600;
  Serial.print("Hour: "); Serial.println(Hour);
  val = Hour * 3600;
  Serial.print("val: "); Serial.println(val);
  Minutes = sec - val;
  Serial.print("Minutes: "); Serial.println(Minutes);
  Minutes /= 60;
  Serial.print("Minutes: "); Serial.println(Minutes);
  minLsd = Minutes % 10;
  Serial.print("minLsd: "); Serial.println(minLsd);
  minMsd = Minutes / 10;
  Serial.print("minMsd: "); Serial.println(minMsd);

  Serial.print(Hour); Serial.print("h"); Serial.print(minMsd); Serial.println(minLsd);

Serial port output:

Hour: 9
val: 32400
Minutes: 3599
Minutes: 59
minLsd: 9
minMsd: 5
9h59

Hour: 10
val: 4294937760
Minutes: 65536
Minutes: 1092
minLsd: 2
minMsd: 109
10h1092

2
  • In the Arduino IDE, use Ctrl T or CMD T to format your code then copy the complete sketch.
    Use the < CODE / > icon from the ‘posting menu’ to attach the copied sketch.
3

Apart form

void setup() {
  Serial.begin(9600);

Above it, and

}

void loop() {
  // put your main code here, to run repeatedly:

}

There isn't anything else.

4

Consider dividing seconds by 60 to get minutes, before printing.

5

Run this:

char tBuf[20];

void setup() {
  Serial.begin(38400);

  long sec = 3599;
  int minutes = (sec/60)%60;
  int hours = sec/3600;
  sprintf(tBuf,"%02d:%02d",hours,minutes);
  Serial.println(tBuf);

  sec = 3600;
  minutes = (sec/60)%60;
  hours = sec/3600;
  sprintf(tBuf,"%02d:%02d",hours,minutes);
  Serial.println(tBuf);
}

void loop() {
}
6

This

val = Hour * 3600;

overflows the sixteen bit maths when Hour is 10.

Use an online regular calculator and evaluate

2^32 - 4294937760 - 65536

or

2^32 - 4294937760 - 2^16

to begin explaining the big number that got printed. I just posted that in the browser URL window and google calculate gives the answer.

Do the calculation using 3600 as an unsigned long:

val = Hour * 3600ul;

a7

1 Like
7 

void setup() {
  Serial.begin(38400);

unsigned long sec = 3599;
  pRint(sec);

  sec = 3600;
  pRint(sec);
}

void pRint(unsigned long Value){
  static char tBuf[20];
  byte minutes = (Value/60)%60;
  byte hours = (Value/3600)%24;
  sprintf(tBuf,"%02u:%02u:%02u",hours,minutes,Value%60);
  Serial.println(tBuf);
}

void loop() {
}
8

That's perfect, thank you.

9

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