Connecting DS3231 RTC SQW pin to Uno R3 - preventing RTC battery from being charged

Other Hardware 3rd Party Boards
1

I want to use a DS3231 real time clock module to send 1Hz pulses into an Arduino Uno R3 digital interrupt pin 2. The RTC module is a no-name brand. It says HW-084 on the PC board, but I can’t track it to any manufacturer.

In reading around the subject, I found “The DS3231 SQW is open drain, so you need a pullup resistor to the Arduino Vcc”. But a couple of other posts vaguely mentioned something to the effect that you need to modify the RTC to prevent it from charging the battery (if you are using a pullup resistor).

Now I’m totally confused. I have some knowledge of electronics (I am a radio ham), but I am by no means expert. Can someone please explain what the situation is. And why couldn’t I just use the Uno’s internal pullup resistor?

Many thanks

2

On some DS3231 modules, the power circuit contains an error, so the battery is always under supply voltage. In such modules, it is advisable to remove the part of charging circuit to prevent damage to the battery.

As far as I know, this has nothing to do with the SQW pin.

See this thread for details:

3

Says who?

4

It depends on if you are using a real DS3231 or the ZS-042 which has a few problems. Have a look at one of the Chronodot modules or work with the naked DS3231 chip itself. I have a bunch of Chronodot's, some V1, some V2 and some V2.1, Haven't found a use for the V3 yet. I provide links to the maker and a 2nd good supplier but they are easily available on Amazon, you just may not get the version depicted but that isn't normally a problem.
Maker look at menu on left side for V2 and V1
2nd supplier

5

Often the module will have a pull-up resistor for the SQW output. For normal operation this is fine, but if you want to minimize power consumption this needs to be disconnected so that an external pull-up can be used without supplying power to Vcc of the RTC. This allows the RTC to run off its own battery the majority of the time, only needing the Vcc power when you want to read/write to the RTC. The RTC needs very little current from Vcc, it can be powered by a HIGH level on an output pin of the Arduino, making it easy to switch power ON/OFF.

6

Thank you b707. It's a great thread.

7

Hi sterretje
All the threads mention installing a pullup resistor. None mention enabling the internal pullup resistor of the Uno. I just thought there may be a reason I'm unaware of.

8

Thanks David_2018