Peculiarities with I2C RTC Clock (DS3231 based)

[xanatos]

First, let me start by saying that everything IS working.

But here's the interesting part: It shouldn't be working. There is no power to the I2C device!

I'm using the CircuitBrains Deluxe (sweet little module!) with a generic I2C based higher-precision RTC using the DS3231 chip. I have it set up and displaying the time & date correctly on my 1.9" TFT display (also another sweet little item!) The kicker is, however, that when the RTC's Vcc (spec'd on the site where I bought them from as being 5.0 volts) is hooked up to the 5V power rail in my project, the microcontroller doesn't boot, at all.

When I disconnect the Vcc line from the RTC and reapply power, everything boots. AND THE TIME DISPLAYS CORRECTLY WITH NO POWER TO THE RTC! How can this be? I have SCL, SDA and GND hooked up, and Vcc with nothing connected to it, and the chip is working and keeping time and responding to the controller's request for the time update.

I have my suspicions as to whether the Vcc for this RTC should be 3.3Vdc - because when I hook Vcc to 3.3Vdc everything boots fine AND the RTC continues to work.

But for the life of me I cannot understand how the RTC is running without a Vcc unless it is running from the backup battery cell... or somehow sourcing power from the I2C lines...???

Truly curious. For anyone wanting to view the "specs" for the RTC module, here's the archived eBay listing: https://www.ebay.com/itm/115345166041?n ... 7675.l2557

Thanks for any thoughts on how a module can work without power lol

Dave

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[adafruit_support_carter]

But for the life of me I cannot understand how the RTC is running without a Vcc unless it is running from the backup battery cell.

That's it. The RTC maintains time using the battery when not powered externally.

[xanatos]

So then I'm guessing that the Vcc = 5V spec is probably incorrect for this module - probably should be 3.3V since nothing boots when RTC Vcc = 5V, but everything boots when Vcc = 3.3V (or... 0V lol)

I just thought the battery only kept the timekeeping counter working, and not the entire module including I/O communications. Interesting.

Thanks for the comment - I just wanted to run it by others with possibly more experience to see if there was some oddity I was unaware of :)

Dave

[adafruit_support_carter]

VCC and VBAT are different.

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[xanatos]

That's what causes me the question - it just surprises me that the IO ports (I2C ports) respond to requests when on Vbatt. I thought Vbatt made it so that everything BUT the clock's counter were shut down to conserve the backup battery's charge.... but apparently not on this module...

Thanks for the detail, useful info.

Dave