DS1307 real time clock is working too slow

[thuvu]

ds1307 not ocillate

Hallo,

I am working with DS1307 and I am trying to create with AVR,LCD a clock.
The problem is that every few hours , I notice that it is 1-2minutes slow.

What is the problem? I am using the crystal which is proposed by the manufacturer for DS1307.

Thanks

 

[IanP]

Re: DS1307 real time clock

If it was one second (or less) every day, I woudn't bother, as you can't do much about that (that's the beauty of this IC) ..

However, in your case, you have the following options:

- replace the DS1307 .. possible solution ..
or
-replace the crystal .. very unlikely solution ..
or
double check your PCB layout around the DS1307-crystal area .. the most likely reason/solution ..

Regards,
IanP

 

[abu_zakan]

Re: DS1307 real time clock

You can get such a problem in case you supply vbat above 3.5v. If this is the case then try not exeed 3v.

I had the same.

 

[philba]

DS1307 real time clock

There are several reasons for clocks/oscillators being off.

- wrong load cap crystal used. as I recall, the 1307 wants 12.5 pf. If you used, say, a 6 pf crystal, it would oscillate at a different frequency and your timing would be off. I suspect this is your problem. there is an application note on maxim site about dallas RTCs and crystal accuracy : application note 58.
- noise near the crystal pins. it's quite common to see this. the noise will get counted as oscillations and the timer will run fast. the datasheet has good info on layout.
- parasitic capacitance. did you build in a "guard ring" around your crystal traces?
- are you using a solderless breadboard? if so, it is going to be very hard to solve the problem because of the large amount of capacitance and noise that it encourages.

Phil