millis() is innacurate on my arduino.

Forum 2005-2010 (read only) Software Troubleshooting
1

I'm new to Arduino and am learning as I go. Today I got my 16x2 parallel LCD working and tried to run the following code:

#include <LiquidCrystal.h>

LiquidCrystal lcd(11, 13, 12, 7, 8, 9, 10);

int i;

void setup()
{
lcd.clear();
}

void loop()
{
lcd.home();
lcd.print(millis());
}

It displays just fine, and counts up. However, I've timed it and in 5 minutes, it's only counted up to ~200,000 (vice 300,000.) It's 2/3 slow every time I try to run it.

Why is my arduino timing things slow? Have I done anything wrong?

-Brady

2

From wiring.c :

/* Delay for the given number of microseconds. Assumes a 8 or 16 MHz clock.

  • Disables interrupts, which will disrupt the millis() function if used
  • too frequently. */
    void delayMicroseconds(unsigned int us)

And, looking inside the LiquidCrystal.cpp :
void LiquidCrystal::home()
{
command(0x02); // set cursor position to zero
delayMicroseconds(2000);
}

Your sketch is calling that home() function constantly so there's a good chance that the controller will be in the delayMicroseconds() routine with interrupts disabled when the counter overflows...

You might try replacing the home function with:
void LiquidCrystal::home()
{
command(0x02); // set cursor position to zero
unsigned long waitUntil = millis() + 2L;
while( millis() < waitUntil ) ; // Block until you've waited the minimum delay here.
}

I don't have an LCD to try this out with but the following sketch looks happy compared to my PCs clock

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

void loop()
{
waitASec(5000);
Serial.println(millis(), DEC);
}

void inline waitASec(int i)
{
unsigned long timeToWait = millis()+i;
while( millis() < timeToWait ) ;
}

3

That's pretty good debugging, Fjornir! :slight_smile:

I encountered a similar problem when monitoring serial GPS data. Whenever I used lcd.home() or lcd.clear(), I would start dropping characters from the software serial port. It turned out to be exactly the same problem.

Thanks,

Mikal

4

Okay, I tried running your sketch and checking millis() in serial. Works fine, 5 minutes had 300,000 milliseconds.

I added in the changes to LiquidCrystal.cpp, and it made no difference. (Is there another step I need beside just making the change and saving the file?)

But I did try:

#include <LiquidCrystal.h>
LiquidCrystal lcd(11, 13, 12, 7, 8, 9, 10);

void setup()
{
lcd.clear();
}

void loop()
{
waitASec(5000);
lcd.home();
lcd.print(millis());
}

void inline waitASec(int i)
{
unsigned long timeToWait = millis()+i;
while( millis() < timeToWait ) ;
}

And that seems to be working perfectly. I'm still not sure what the original problem was, but at least I know I don't have a "slow" arduino! Thanks Fjornir!

5

If you modified LiquidCrystal.cpp to not call into delayMicroseconds() when you're using the home() method then you might need to delete the LiquidCrystal.o file (I hear the arduino environment is a bit bad about detecting changes to library files) before asking it to rebuild your sketch.