Problem with Real Time Clock (DS1307)

[yviswanathbe]

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

 

[goldberg]

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