Hello All !

May I first say that I am very excited to have discovered the Arduino. I became aware of it through an article in the Linux Format magazine in the UK (I think they are hoping to run more articles soon). I have come across, and used, a few microcontroller platforms through my work. Whilst these development kits are useful at work, I've never seen anything I'd liked enough to spend my own money on, for my own personal use at home (expensive kit, expensive compilers, etc). That was until I discovered the Arduino! I really like how simple it is to program ... no complicated configuration of the low-level devices. I'd like to congratulate everyone involved, what a great job!

Now to my question ...

I have a project idea where I'd like to implement a clock. A simple 24hr clock is all I need ... no date is needed. It doesn't have to be incredibly accurate. I could probably put up with one or two minutes drift a month if that is the best that can be acheived.

I have searched this forum the best I can but the only ideas I have found are to interface to a real-time clock chip. However, I was wondering if this could be done entriely on the Arduino? I was thinking that I could do something with either (a) a hardware timer and an interrupt service routine or (b) use the millis() function in some way. [I have another question about the millis() function but I'll post that separately.]

Can someone advise me if this is possible? If not, then what are the problems (e.g. accuracy)? If it is possible then maybe someone has come across a project or tutorial I could reference?

Any help will be most gratefully received.

My best regards to everyone .... Matt

Yeah its easiest just to get a simple RTC.
A small chip and a crystal gives you very accurate time for under a dollar.

Thanks for all the replies, I wasn't expecting any so soon.

Whilst I appreciate that a separate real-time clock IC will the most accurate solution, I think I'll have a go at doing it on the arduino first ... more for the challenge of it than just to be difficult.

My design will have a keypad and a LCD, so there will be a mechanism to set the clock ... I think I'll also introduce a 'fiddle-factor' to give me a certain amount of tuning.

In case anyone is interested, my design is for a controller to for my fish tanks (I keep freshwater and a small marine). The main thing it will do is control the lights, there are 3 sets; daytime, dusk/dawn and nighttime, which go on and off at different times. I currently use plug-in mechanical timers at each socket. These are cheap but take a bit of setting up to get the lights to sequence as you want them to. There are commercially available controllers but they are expensive (£200 min). My idea is to build something from an arduino (say ~£25). Then if I'm not happy switching the mains electricity directly, I'll set it up to do X10 automation. I'll also pull in temperature monitoring, and maybe a system to run an automatic water top-up system (using float switches and a small water pump) ... saving even more money. I guess projects like these are more more about the hbby factor than purely money driven.

As for the clock thing, I'd be happy to share whatever I come up with, for interest and comment. Maybe even have a go at building a library? I'm not a strong programmer so can't promise super-efficient code but if it helps, I'm happy to participate.

Thanks All ..... regards ..... Matt

Maxim DS1307 http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2688 is an 8pin DIP RTC that uses a tiny standard clock crystal. If you do a search on the forums and also in the playground there are code and examples on how to use it. You can order samples from maxim for nothing and I got my crystals from toys in cornflakes boxes(no joke!) and old motherboards. Just look on any toy or old motherboard for a small round metal can that looks similar to an electrolytic capacitor, thats usually the crystal.

I have been playing around with this clock script.  Its not very accurate but could be used to control things measured in hours or days.  Every time you add to the script, you will have to recalibrate the clock, which takes a few days to zero in.

Code:

//  Variable clockadjust will be about 1000 milliseconds per second
//  If clock lags realtime, decrease clockadjust
//  If clock preceeds realtime, increase clockadjust
//  http://www.time.gov
//  Adjust variables to desired set time
//  Press reset two seconds prior to desired set time

int clockadjust = 986;
int day = 0;
int hour = 21;
int min = 36;
int sec = 0;

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

}

void loop()
{
delay(clockadjust);

  sec = sec + 1;

if (sec > 59)
{
  sec = 0;
  min = min + 1;
}

if (min > 59)
{
  min = 0;
  hour = hour + 1;
}

if (hour > 23)
{
  hour = 0;
  day = day + 1;
}
  Serial.print("Day ");
  Serial.print(day);
  Serial.print(" - ");
  Serial.print(hour);
  Serial.print(":");
  Serial.print(min);
  Serial.print(":");
  Serial.print(sec);
  Serial.print("\n");
}

 

I also found delayMicroseconds() - http://www.arduino.cc/en/Reference/DelayMicroseconds .

That should allow for fine tuning of the clock.  

I have the current clock script accurate to about 1 second per hour.

Does millis() start over every time the loop cycles?  Or does it continue counting from when the loop starts the first time?

To combat the rollover you have another variable which you put millis() in every time you use it.
The you compare the current millis() to the stored variable. If its smaller then you work out (4294967296 - stored_millis) + current_millis to compensate.

skyl4rk wrote on 25.01.2008 at 04:19:16:

It uses I2C so you'll be wanting to look at the Wire library to communicate with it.
You hook it up to the SDA and SCL pins. On the Arduino they are Analog pins 4 and 5 respectively.

More info can be found by searching.