The most accurate clock in the building besides everyones cell phone.

The common cell phone typically uses data from atomic clocks provided by the cell service. However my IceCube clock found on http://www.adafruit.com is much more enjoyable to read the time from. The clock comes as a kit and employs a Russian VFD to display the time. Yes, supercool I know. The time keeping is done with the heartbeat of a standard 32.768 Kilohertz watch crystal. I have replaced the standard crystal in my clock with a Chronodot RTC. This is a Real Time Clock module with lots of go fast stuff built in and an I2C interface. Even a back up battery. I didn’t use any of that fancy stuff. All I cared about was the Thermally Compensated Crystal Oscillator (TCXO) onboard. This could have been done with any TCXO but I had this one and it was already running at 32.768 Khz. the 32K pin on the above left image gets pulled to ground for every pulse. To use this, one gets a pull up resistor. I used a resistor around 100K Ohms. After removing the watch crystal and a couple of filtering caps I soldered the input pin of the clock to the 32K pin of the Chronodot then soldered the 100K resistor to the 32K pin as well. Then I connected the resistor to plus 5 volts. This limits the current going to the clock but more importantly when the Chronodot pulls the pin to ground, it limits the current preventing a short circuit.