Problem with Real Time Clock (DS1307)

Hi,

I am doing a Digital Timer using AT89C4051 and DS1307 RTC. Time is displayed on 16x2 LCD.
I have enabled 1 HZ square wave output and updating the time on LCD every rising edge.

The problem i am facing is the RTC time is not accurate. For every 24 hours i observed a delay of 2 minutes. I have assembled two other boards and checked, I have grounded the crystal body, replaced the crystals(Crystal Frequency-32.768kHz), but the problem is same.

I have measured the crystal frequency, 1 HZ pulse with the oscilloscope and it is fine.

What could be the problem? Can somebody guide me to solve my issue.

Thanks and Regards,
Viswanath

Viswanath,
There might be some capacitors or a very small trimmer around the crystal, try to check the tolerances or change the capacitors.
2 min in 24h is too much. May be you get some other cheap 32768 crystals on ebay and check which one works better.
Oh, the trimmer should be 2-20pF max, If you have a frequency counter check the frequency as a counter and see if you are bad
within 1 min, that is 32768 Hz x 60= 1966080 periods. Check it out with a high impedance probe. If you calculate the 2 min/24h fail down to
1 sec actually you will have 32735 Hz instead. Pls check that also. Lower frequency normally shows a bigger capacity drift.
This is also an issue of older crystal wrist watches which I try tor repair if possible.

Also, please check the specification of the DS1307: ###Accuracy > 2 minutes/month at 25 C ###... (https://www.futurlec.com/Dallas/DS1307.shtml)
Just a simple output on pin X1 an X2, without capacitor to trim the frequency.
Lets see how it works, good speed!
Thomas