RTC 32.768Khz 6pf -v- 12pf

Got one!!

Rapid;

When I used a 12 pf crytal with a DS1202, it gained a minute a DAY.

--Gene

As a point of interest, note that PC motherboards used to have a trimcap on the 32768Hz RTC xtal to tweak the timebase. Several minutes per day were available through a small (

I think this is because nobody really cares about the RTC accuracy. Besides, it is lot easier to correct the time count digitally, rather then tweak the trimmers.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

Take a look at any 486-class or earlier Pentium motherboard. I am much too lazy to take photos right now!

I'm not aware that the RTC macrocells embedded in super I/O chips include such tweaking functionality.

I just depopulated an 8MHz XT-clone M/B which had the trimmer; I was earlier going to suggest specifying a rock high in freq at 12pf and add the trimmer.

Most XT-clones with RTC and AT clones had the trimmer.

Regards,

Michael

One reason for low availability of very small capacitors is that the stray capacitances will dominate anyway. If everything else fails, put to 12 pF capacitors in series to get 6 pF nominally, but again the strays may cause problems.

If there are some free PCB space, making a 6 pF capacitance from just the stray capacitance between the PCB tracks is not hard. This could even be adjusted by cutting out or adding sections, which can be useful if the required load capacitance varies from one crystal to an other.

If the capacitance is from crystal terminal to ground, a multilayer PCB with a ground plane, the capacitance can be formed between a PCB track and the ground plane through the PCB dielectric, but unfortunately some PCB materials are sensitive to humidity and the capacitance may vary.

A DC blocking capacitor and a capacitance diode could also be used if voltage control is required, but there are temperature coefficient issues with capacitance diodes.

Paul

That's off allot more than a few pF of capacitance is going to make. These things are prone to picking up stray AC noise and counting it. At 60Hz, it could add as much as 158 seconds per day, 132 seconds at 50Hz. When I was tinkering with some on a breadboard, I had to ground the crystal case to get it to keep good time. Using smaller caps probably increased the amplitude of the oscillator perhaps drowning out the AC noise.

These

it

Also picked up off the net is that grounding the can case can limit odd behaviour.

Thanks Paul :-) Acknowledged.

For anyone reading this thread in future, also search on similar Maxim part numbers;

DS1305 DS1306 DS1307

Each will bring back different hits but are sufficiant for sourcing useful information from a personal DIY point of view.

Are you sure the datasheet says "what the RTC is designed for"? I would expect not. The crystal has been specified to run on its frequency with a certain load, not the RTC.

Taking the first hit I find with google, the motional capacitance of a watch crystal is 2fF. Putting 12pF in series wil give about 7 seconds change per day. Putting 6pF in series gives 14. So the difference between 12p and 6p will be about 7 seconds per day.

If your crystals are specified for 12p, just place 2 times 22p (or better 22p and 27p) at the crystal. I would be really surprised if the RTC would not work. You might have to substract the port capacitance of the RTC itself (order of 3p?).

Cheers,

Joop

Aly" a écrit dans le message de news: snipped-for-privacy@bt.com...

Hi,

Have a look at DALLAS app note : "Application Note 58 - Crystal Considerations for Dallas Real-Time Clocks"

Yvan