Posted 30 October 2020,

Back in May of 2018, well over 2 years ago, I posted about adding an Adafruit DS3231 RTC module to Wall-E2, my autonomous wall-following robot project. This addition went swimmingly until about 6 months later in September of 2018 when I posted to the Adafruit support forum, saying that I was having trouble with the ‘lostPower()’ function return values; it seems like it was returning FALSE (no power loss) even though I had removed the battery and turned off the power to the system. As described in the post, I eventually gave up on this in February of 2019after discovering that I was getting radically different results when I used a different Arduino Mega and two different Adafruit DS3231 modules. Eventually I wound up in the situation where both DS3231 modules appeared to work correctly no matter what I did – strange!

Fast-forward to the present. In the process of adding a rear distance sensor to Wall-E2, I once again ran across the same anomalous behavior by the Adafruit DS3231 RTC module; The ‘lostPower()’ function stubbornly refused to declare a loss of power, even with the battery removed and the main power turned off. After a lot more investigation, including a dedicated test program and some more back-and-forth on the Adafruit forum, I (and the Adafruit support guys) still was unable to resolve the issue.

In desperation, I fished a generic ‘ZS-042’ DS3231 RTC module out of my parts bin and started working with it, thinking maybe I could use it to get a clue why the Adafruit modules were failing. As it turned out, the ZS-042 module worked perfectly from the get-go with the Adafruit RTC library, and the ‘lostPower()’ function correctly returned TRUE when main power was lost with the battery removed, and FALSE when power was lost but the battery was in place.

Here are some photos of the Adafruit and ZS-042 modules:

As can be readily seen, the ZS-042 module is considerably larger, due almost entirely to the decision to use the LIR-2032 Li-ion rechargeable cell instead of the smaller non-rechargeable CR1220 type. Other differences:

• The ZS-042 module includes a power LED. This LED illuminates when main power is available on the VCC pin, but not when the RTC module is running from the battery
• The Adafruit module exposes the RST (reset) line. If you need this, the ZS-042 won’t work for you.
• When used with the supplied LIR2032, the battery is recharged and/or float-charged from VCC through a 1N4148 diode. This works fine if VCC is 5V, but doesn’t work at all if VCC is 3.3V.
• The 32KHz output is open-drain, without a pullup on both the Adafruit module, but the ZS-042 module has a pullup to VCC. What this means in practice is you can’t easily monitor this output when operating off the battery, so it is hard to tell if the RTC module is still running. My solution to that was to attach a completely separate power supply to the 32KHz output via a 10K pullup resistor. The Adafruit module needs this to see the 32KHz output for both battery power and mains power. The ZS-042 module only needs it for battery power.

So, there you have it. The Adafruit module is smaller, has an additional output (RST) and uses a smaller, non-rechargeable CR2210 button cell. However, in my testing and use over a two-year period, I came to distrust its ability to reliably detect and report on complete power loss situations that would require a forced date/time update.

The ZS-042 module is significantly larger due to its use of the rechargeable Li-ion LIR2032 button cell, and doesn’t have the RST output. It is also considerably cheaper and widely available. Lastly, it appears to more reliably report complete power loss occurrences, allowing proper date/time updates.

For my money, I have replaced the Adafruit DS3231 module in my system with the ZS-042 module. In practice, complete RTC power failure events are very rare, so in all probability there would be no appreciable difference between the two choices. However, for those applications (like mine) where you really do want to know if the RTC loses its sense of time, I don’t feel comfortable with the Adafruit module.

If anyone has a better understanding of the Adafruit module, please feel free to comment.

30 October 2020 Update

I replaced the Adafruit DS3231 RTC module on my Wall-E2 autonomous wall-following robot with the ZS-042 DS3231 RTC module. As shown in the following photos, I had to re-arrange the I2C FRAM and I2C MPU6050 IMU modules in order to make room for the significantly larger ZS-042 module.

Stay tuned,

Frank