I want to install a Adafruit DS3231 RTC breakout on my RPI 3, the problem that I see is that the I2C pins (3->SDA1, 5->SCL1) recommended for installing are not free because I have a RazBerry Z-Wave module installed there.

Could I wire the RTC breakout on pins 27 and 28 and use SDA0 and SCL0?

Or should I search for another RTC module/breakout ?

1 Answer 1


You can't use pins 27 and 28 they are reserved for the eprom used to configure a hat. This can be verified by the note included in the schematic for the 40 pin header.

40Pin Schematic

The HAT design guide includes this:

On a Model B+, GPIO0 (ID_SD) and GPIO1 (ID_SC) will be switched to ALT0 (I2C-0) mode and probed for an EEPROM. These pins will revert to inputs once the probe sequence has completed.

The only allowed connections to the ID_ pins are an ID EEPROM plus 3.9K pull up resistors. Do not connect anything else to these pins!

While the above specifically mentions the model B+ this is true for all Pi's with a 40 pin header.

However, the I2c bus supports multiple devices as long as they have differrent addresses (this is likely the case with the two different devices you have - though most devices also support changing the address). So your problem really comes down to getting access to the I2C pins which are inaccessible due to your Z-Wave module. A board like this one

Mini Black HAT Hacker Board

can allow you to access the I2C pins which are inaccessible.

You can also remove the Z-Wave and connect it and the RTC to the Pi using a breadboard and jumper wires.

  • What about 3.3V versus 5V boards? I noticed that the RTC modules (RTC Pi Plus and RTC Pi Zero) from AB electronics UK have level shifters so that other I2C boards can be attached on top. I would suspect that the Adafruit board cannot be stacked.
    – TheDiveO
    Commented Jul 20, 2017 at 8:15
  • @TheDiveO Here is a quote from the adafruit page learn.adafruit.com/adafruit-ds3231-precision-rtc-breakout/… "Vin - this is the power pin. Since the RTC can be powered from 2.3V to 5.5V power, you do not need a regulator or level shifter for 3.3V or 5V logic/power. To power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V micro like Arduino, use 5V" It also does not need to be stacked attacj it t the hacker board the same way you would attach it to the GPIO pins that is what the board does. Commented Jul 20, 2017 at 12:07
  • I will try to find a way to access the same GPIO pins for both modules but also be able to keep the current compact case, a breadboard would leave me with a cable mess, but the mini black hat looks promising. Thanks for the answers!
    – Nicolae
    Commented Jul 20, 2017 at 16:24
  • if size is an issue they just released this one: shop.pimoroni.com/collections/new-products/products/… and there are other similar products. As for the case you could avoid the issue and pickup a food storage container from the local dollar store - they are cheap and easily modded with basic tools. Commented Jul 20, 2017 at 16:30

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