The Precision RTC Plug is a small (21x35mm) board containing a low power real-time clock with battery backup. It continues to track the time with second by second resolution while the attached node is sleeping or even powered off (with optional battery inserted).
An I2C bus running at 3.3V is used as interconnect, with all the power and signal lines brought out to to both sides of the board to allow daisy-chaining with other I2C-conformant boards.
The support library is the same as for the lower specification RTC plug. This board achieves an order of magnitude better apparent drift error by internal compensation techniques that automatically adjust for temperature changes and crystal ageing.
The Assembled Precision RTC Plug comes with the SMD-sized DS3231 chip, crystal, battery holder, coin cell battery, and decoupling capacitor already soldered onto the board. Headers for this and all other JeeLabs plugs can be found here (straight) and here (right angle).
The PCB Only version contains just the printed circuit board for the Precision RTC Plug. No parts, no headers, no battery.
The documentation and full design details are freely available.