32.768kHz Crystal for DS1307 - clock is not ticking

[yogeshc]

Hi All -

I am using DS1307 RTC to make a digital clock. In order to run it I am using 32.768kHz crystal and displaying time on LCD. Everything is working fine. I am able to set the time and date. But only problem is that clock is not ticking.

I simulate the circuit in Proteus 7.. Where it works perfectly.
I am using cylindrical crystal and not using any capacitors across it.
Also, I used P89C51RD2 as uC in the circuit.

Please let me know how can I fix this proble??
Do I need to put capacitors across crystal ??
If yes, what should be the value of those capacitors??

Thanks in advance.

~yogeshc

 

[astro1]

yes you need two caps, try 22pf

 

[yogeshc]

Thanks astro1...
I'll check if it runs...

 

[dr pepper]

Look up the data sheet for the xtal, cylindrical watch types have various load capacitances, and some operate in series mode.
If a pair of caps doesnt work, try a 1 megohm resistor accross the xtal, this will bias the oscillator gate and encourage oscillation.

 

[klystron]

Measure the oscillator clock signal with an oscilloscope .(10 Mohm probe)

 

[RCinFLA]

32.768 kHz watch crystals are almost always parallel mode requiring between 5 pF to 20 pF of parallel load capacitance to run at correct frequency. The most common load capacitance is 10 pF which would be a 18-20 pF cap on either lead to ground. The more load capacitance required, the higher the oscillator current drain needs to be to drive it.

Most cheap battery backup clocks run slow because the couple of pF's PCB and IC stray capacitance is not taken into account. When in doubt guess on low side (two 18 pF caps to ground node). (18 pF + 2 pF stray) / 2 = 10 pF across crystal.

 

[andre_luis]

I worked with that DS1307 RTC very time ago ( more than 5 years ), and I don´t remember exactly the reason why didn´t worked at first time. But, I remember that some access may be done in protocol comunication to start oscilator.

See that informatio at datasheet :

Please note that the initial power-on state of all registers is not defined. Therefore, it is important to enable the oscillator
(CH bit = 0) during initial configuration.

https://www.sparkfun.com/datasheets/Components/DS1307.pdf

+++

 

[yogeshc]

Thanks all.
I appreciate your quick reply to my post....