ChronoDot V2.1 High Precision RTC
The macetech ChronoDot RTC is an extremely accurate real time clock module, based on the DS3231SN temperature compensated RTC (TCXO). It includes a CR1632 battery which should maintain timekeeping for 8 years if the I2C interface is only used while the device has 5V power available. No external crystal or tuning capacitors are required.
The DS3231 has an internal crystal and a switched bank of tuning capacitors. The temperature of the crystal is continuously monitored, and the capacitors are automatically adjusted to maintain a stable frequency. Other RTC solutions may drift minutes per month, especially in extreme temperature ranges...the ChronoDot will drift less than a minute per year. This makes the ChronoDot very well suited for time critical applications that cannot be regularly synchronized to an external clock. Here is a great article showing how accurate the ChronoDot's DS3231 can be.
Our latest version of the ChronoDot uses the DS3231SN, which is rated for the industrial -40C to +85C temperature range. When operating in the 0C to 70C temperature range, accuracy is +/- 2.0ppm. When operating outside this range, accuracy only drops to +/- 3.5ppm.
Since the ChronoDot needs to read the temperature, it also contains a fairly accurate temperature sensor, accessible through I2C. Stated accuracy is +/- 3C, though the temperature register returns values with a resolution of 0.25C. Calibration may improve accuracy.
The ChronoDot will plug into a standard solderless breadboard and also has mounting holes for chassis installation.
The I2C interface is very straightforward and virtually identical to the register addresses of the popular DS1337 and DS1307 RTCs, which means that existing code for the Arduino, Raspberry Pi, STM32, ESP8266/ESP32 and other controllers should work with no modification.
The ChronoDot V1 and V2.1 design files are currently released under Creative Commons Attribution Share Alike 3.0, until a suitable license under the OSHW Definition is available. You are free to use the Eagle schematic and PCB design included in the files below, in accordance with the included license.