A question about capacitors used with the DS1307 Real Time Clock.

ElectricAye

Hey all,

I'm planning on using the DS1307 real time clock (RTC) and I noticed it requires two 12.5 pF capacitors. Anybody know what kind of capacitors these should be? Ceramic? or what? I'm concerned that if I pick the wrong kind, it will be more susceptible to thermal fluctuations. Any suggestions?

thanks,
Mark

Erik Friesen

It doesn't seem like you can go wrong with an X7R ceramic.

ElectricAye

Actually, I'm very confused about something here. I think the capacitors are already internal to the DS1307 chip and I thought they needed to be supplied externally???

My confusion stems from page 188 of the StampWorks Manual, version 2.1, in "Using a Real Time Clock." There, it shows the capacitors as being external, but the DS1307 datasheet shows the capacitors as being internal and says nothing about adding other capacitors to the outside of the chip.

Anybody know what is the correct situation?

thanks,
Mark

Beau Schwabe

ElectricAye,

The load capacitance of the 32.768kHz cystal should have an internal load capacitance of 12.5pF, but you shouldn't be adding any additional cap.

Reference:
http://www.fuw.edu.pl/~pablo/s/docs/ds1307.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

ElectricAye

Thanks Beau, you just saved me from making another panic-stricken order to Digikey. But since that is the case, you might want to look at page 188 of the StampWorks Manual, version 2.1, in "Using a Real Time Clock." Maybe that schematic needs to be corrected???

thanks,
Mark