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I recently purchased a DS3231 RTC module to use in one of my prototypes. Now that it is complete I would like to integrate the RTC directly into my PCB instead of using a plug in module. To that effect I have been trying to figure out the circuit using continuity testing on the module and the recommended circuit in the DS3231 datasheet. I have figured out all of it, except 2 things.

  1. There are 2 capacitors in parallel connected between VCC and GND. There are no marking on them so how do I figure out their value?
  2. There is a diode - resistor (210 ohm) pair between VCC and ground. Can't figure out the use of these since they are not in the manufacture suggested circuit diagram as well.

Any help is appreciated !

Edit: The diode in question can be seen in this closeup picture at the bottom side of the RTC here

enter image description here

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  • \$\begingroup\$ Does the diode look like it may be a LED to show power is applied? \$\endgroup\$
    – PeterJ
    Oct 17, 2014 at 11:56
  • \$\begingroup\$ @Peterj there is a separate smd led + resistor pair on the module connected between VCC and GND \$\endgroup\$
    – Ankit
    Oct 17, 2014 at 12:10
  • \$\begingroup\$ Not sure what that would be about then. But the caps would just be decoupling, for something fairly low speed / current like that you'd probably just need a 0.1uF. Often proto boards include a second higher value to compensate for a bit of extra voltage drop over long / dodgy prototyping leads. \$\endgroup\$
    – PeterJ
    Oct 17, 2014 at 12:20

3 Answers 3

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Instead of blindly copying what is on the module, get the datasheet for the part and create your own circuit. That will take less time than reverse engineering one particular implementation, and you end up with something you actually understand and know that it meets all the specs. You don't know what kind of tradeoffs went into designing the module, and you don't know how competent the designers of the module were.

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  • \$\begingroup\$ I totally agree and that's why I did make my circuit using the manufacture recommended in the datasheet. The exercise with the module reversing is just to verify my understanding. The one created through the datasheet is exactly the same as the one obtained by reverse engineering the module (except for the diode) \$\endgroup\$
    – Ankit
    Oct 17, 2014 at 13:20
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Found it. As per the posts here and here, the diode/resistor are connected between VCC and VBAT (that was my bad, I though they were connected to GND). This apparently is used to trickle charge the module with LIR2032 rechargeable battery. So in my case, I can ignore it.

Thanks all for the help !

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The 200 ohm resistor and diode are for charging a LIR2032 rechargable LiIon battery. I would not recommend using rechargable battery though. If you do, you need to keep the Vcc at 3.8 to 3.9 vdc to avoid overcharging LIR2032 battery. A full charging is 4.2vdc but you need to drop voltage after battery is fully charged.

Remove 200 ohm chip resistor and stick with CR2032 non-rechargable battery. Setup DS3231 so it turns off the 32 KHz clock output to reduce current drain from backup battery when Vcc is turned off and keep EOSC running when Vcc is less then Vbat (Vcc turned off). Battery will last a long time. CR2032 battery is 3.0 vdc and you do not want to charge it so you need to remove 200 ohm resistor.

enter image description here

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