The RTC Plug uses a DS1340 chip to keep track of time. It has its own on-board coin cell and 32 KHz crystal, so that it can keep running when power to the JeeNode is switched off.
The 32,768 Hz crystal is rated at 20 ppm, with 3 ppm aging in the first year. A year has about 3600 * 24 * 365.25 = 31557600 seconds. With 20 ppm, the RTC Plug can be some 630 seconds off at the end of the year, i.e. close to one minute per month – worst case, that is. It’s fine for lots of uses, but evidently not all…
Here’s a new plug prototype, designed by Lennart Herlaar:
It’s based on the DS3231 chip, which includes a “TXCO”, i.e. a Temperature Compensated Xtal Oscillator. There are many ways to improve the stability of a crystal oscillator, but this one is pretty nifty because it’s low-power (unlike an oven-based technique, which puts the crystal in an controlled temperature environment). The idea is that the chip periodically measures the ambient temperature, and then adjusts the capacitive loading of the crystal according to a calibrated function of temperature. IOW, it knows how to compensate its on-board crystal.
The DS3231 is specified at 2 ppm for temperatures 0 .. 40°C, i.e. ten times more accurate than the above crystal. So this clock will stay on time with an error of at most one minute per year, i.e. about a second per week.
For something, eh… slightly better – you could consider a Rubidium Frequency Standard with 50 ppt accuracy, i.e. well under a millisecond per year error. But don’t expect to run it off a coin cell – it draws about 11 Watt :)
If there’s enough interest, this plug could be added to the shop…