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DS3231

The DS3231 is a cheap and widely available i2c RTC module available in various form-factors. It uses i2c address 0x68.

My favorite variant is one that fits on the Pi2+ header and includes a soldered-on rechargeable battery.

Requirements

i2c bus with address 0x68 free.

Usage

You need to know the name of the i2c bus as described in the devices device-tree. You can consult your devices device tree or poke around /sys/firmware/devicetree/base/aliases/.

The i2c bus must be enabled, status = "okay", this will currently be forced due to how the DT simple-overlay is structured, but is not intentional, expect it to change and enable your i2c bus if it is not already enabled. The board contributor should have defined user-exposed i2c interfaces at sbc.board.i2c.devices.*.

Finally, enable the dtOverlay with:

host = nixosSystem {
  modules = [
    # Note: The deviceBuilder API should be considered unstable.
    (nixos-sbc.deviceBuilder.rtc.ds3231 ({config}: {i2cConfig = config.sbc.board.i2c.devices.i2c0;}))
  ];
}