RTC draining a coin cell battery in one day?

[Zapro]

What is the exact part number on the battery? LIR2032?

I ask as if this is a module that already came with an LIR2032, there is a potential discharge circuit on the module. It is meant to charge an LIR2032, but pretty much kills CR or BR versions. There is a resistor you can remove to the battery charge circuit if you want to use a normal battery not a rechargeable.

Reading the first post from OP doesn't indicate this:
"OP: I've breadboarded a DS3231M RTC using a basic SOIC8 breakout board"

The problem could just as well be a dud chip or conductive flux remains under the chip.

// Per.

[MarkT]

Fresh flux doesn't conduct, unless its badly burnt - flux is normally removed
so that it can't degrade over years/decades and start leaking electrons.

[aarg]

The DS3231 datasheet says,

Avoid running signal traces under the package, unless
a ground plane is placed between the package and the
signal line. All N.C. (no connect) pins must be connected
to ground.

However, I know that it operates without them connected, because I breadboarded a prototype that way. I think I will go back and correct that when I get my better soldering iron back.

Considering that, it's likely that those pins are connected to some kind of internal local ground plane, or more simply that the unused pins are in close enough proximity to the actual crystal connections, that there could be interference coupled into the circuit. Grounding them would effectively eliminate such interference.

It's worth considering the part about adjacent lines as well. Low power crystal oscillators are very sensitive circuits.

Having said that, it doesn't seem to have anything to do with the original question about power drain.

[iamchriskelley]

It's worth considering the part about adjacent lines as well. Low power crystal oscillators are very sensitive circuits.

Having said that, it doesn't seem to have anything to do with the original question about power drain.

Yes, to be clear I took a chip without supporting circuitry and soldered it to an Adafruit SOIC-8 breakout board. The breakout board has traces for an TSSOP-8 chip on the other side -- no ground planes, traces underneath the RTC. All eight pins of the breakout board were connected to independent rows on a breadboard.

After reading the previous comments I decided to use a freshly (and better-) soldered DS3231M and move the breakout board off of a breadboard and use jumper wires to connect the RTC to the Uno and the coin cell. I'm not sure to what extent parasitic losses from the breadboard (especially from the unused INT/SQW, 32KHz, and RST pins) could have affected things; like the traces under the RTC I would expect these losses to affect the ability of the RTC to accurately keep time (they didn't) rather than dramatically affect the battery life.

I'll check on the reconfigured device (and a Chronodot I wired up for comparison) tomorrow.

[aarg]

I'm not sure to what extent parasitic losses from the breadboard (especially from the unused INT/SQW, 32KHz, and RST pins) could have affected things; like the traces under the RTC I would expect these losses to affect the ability of the RTC to accurately keep time (they didn't) rather than dramatically affect the battery life.

No such losses are predicted by the manufacturers data sheet. Nor is there any cautionary note about leaving them disconnected. INT and 32Khz are explicitly permitted to be unconnected. RST has no clear language except that it has an internal pullup (however this would make it safe).

The outputs that are open collector wouldn't draw power from Vcc or Vbat anyway. Not unless you wired them that way.

[TomGeorge]

Hi,

Can you post a picture of your project please?

Tom...

[aarg]

Yes, to be clear I took a chip without supporting circuitry and soldered it to an Adafruit SOIC-8 breakout board.

Well, the 3231 has a lot more than 8 pins. If you put that on the breakout board you posted a link to, some of the pins would come dangerously close to the through holes. Maybe one of those is making contact.

Or did you really mean this one:
https://www.adafruit.com/products/1207

[Zapro]

Well, the 3231 has a lot more than 8 pins. If you put that on the breakout board you posted a link to, some of the pins would come dangerously close to the through holes. Maybe one of those is making contact.

Or did you really mean this one:
https://www.adafruit.com/products/1207

OP is using the DS3231M. It's electrically compatible with the old DS3231, it's just in a small housing.

// Per.

[iamchriskelley]

Here is a photo of the DS3231M I'm using on a breakout board. Apologies for the late reply; I wasn't able to make it to the office yesterday.

I left the DS3231M disconnected from the Uno, and it did not run down the coin cell it was connected to. I'm repeating the test now with the Uno connected to the VCC, GND, SDA, and SCL lines.

[MarkT]

Please crop and reduce images to a sensible screen resolution!

Probable dry joint on the far corner, far too much solder everywhere, flux needs
cleaning off, its drenched!  Smooth concave surfaces on the solder are a sign
things went well, convex surfaces means too much solder (and hard to see
the joint quality).

[iamchriskelley]

Probable dry joint on the far corner, far too much solder everywhere, flux needs
cleaning off, its drenched!  Smooth concave surfaces on the solder are a sign
things went well, convex surfaces means too much solder (and hard to see
the joint quality).

Points taken, my SMD hand-soldering skills aren't great. The joint you mention on the far corner is unattached in the circuit. I inspected both RTC chips I boarded, with a USB microscope, and didn't see any bridges on either board. Of course it's possible I missed one.

[CrossRoads]

Testing adjoining pins with a multimeter will tell you very quickly.

[iamchriskelley]

Testing adjoining pins with a multimeter will tell you very quickly.

Good point -- just checked the leads and no pin pairs have resistance lower than 4MOhm.

[iamchriskelley]

Well, my second DS3231M seems to be working fine. My apologies for jumping the gun -- could have replicated the experiment before posting. The odds of getting a bad chip from Mouser are presumably quite low so I'm not sure what happened here. If anything else odd comes up -- repeatably -- I'll post. Thanks.