Here’s another new board, the Precision RTC Plug – this is is a revision of a design by Lennart Herlaar from almost a year ago – my, my, this year sure went by quickly:
The current RTC Plug from JeeLabs will be kept as low-end option, but this one reduces drift by an order of magnitude if you need it: that’s at most ≈ 1 second per week off over a temperature range of 0 .. 40°C. Or one minute per year.
Drift can go up to twice that for the full -40 .. +85°C range, but that’s still one sixth of the crystal used in the original RTC Plug. Considerably better than this, in fact, if you need the extended temperature range. Here’s a comparison between both plugs, from the datasheet:
The way the Precision RTC works is with a Temperature Compensated Crystal Oscillator (TCXO): once a minute, the approximate temperature is determined and the capacitance used by the crystal oscillator is adjusted ever so slightly to try and keep the 32,768 Hz frequency right on the dot. Since the chip also knows how long it has been running, it can even apply an “aging” correction to compensate this small effect in every crystal.
The temperature can be read out, but it’s only specified as accurate to ± 3°C.
No need to use any special software for this, all the normal clock functions are available through the same code as used with the original RTC Plug. If you want to use fancy functions, or perhaps calibrate things further for an even lower drift, you can access all the registers via normal I2C read and write comands.
The board will be added to the shop in a few days, and the wiki page on the Café updated.