Can the DS1307 lose about 3 seconds in 18 hours?

jaikumar · Joined: 15 Dec 2006 Posts: 109

Hi all,

I made a alarm clock using ds1307. I am losing 3 seconds in 18 hours.
Is it something that happens ? If so what should I do to avoid this ?

Thanks and regards,
jaikumar.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Describe your crystal circuit. Post the manufacturer and part number of
your crystal.

jaikumar · Joined: 15 Dec 2006 Posts: 109

Crystal is directly connected between pin 1 & 2 of DS1307. No Caps used.
Crystal is close to DS1307.
Crystal Spec:

Frequency: 32.768Khz
Freq Stability: -0.034+/-0.006ppm
temp: -20 to 70 degC
Load Cap: 12.5pF
Make: CITIZEN AMERICA.

I am using the computer clock to measure.

Also let me know if the
ds1307_init() is required every time we power up, or only one time to set
via computer. Because i kept losing a second on power up every time.

Regards
Jai Kumar.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Ttelmah · Guest

First, Double check your computer clock. Use one of the internet time servers, and reset this, before taking each reading. PC clocks are often _appalling_. Unfortunately, some software functions in the PC can interfere with the clocks, making things worse, and the crystals used are often very poor accuracy. So, start from an accurate source, before deciding there is a problem with the DS1307. It is common for PC clocks to be several minutes fast in a week. Have a look on the web, and you will find that the PC is not a good timekeeping reference.

Then, assuming there still is a problem, how is the PCB laid out round the crystal legs?. What is the PCB substrate, what clearances have been used etc. etc..
Your crystal 'looks right' for the chip (which is designed to work directly with crystals requiring 12.5pF loading capacitance). Your error, is 1:21600, or 46.3PPM, which is many times worse than you should be able to achieve...
Are you sure that your battery on the DS1307, is within the required voltage range?. There is a common problem, with people trying to use _rechargeable_ nominal 3v lithium cells which can give a voltage above 3.5v on charge....
The init code should not cause a time loss.

Best Wishes