April 1, 2011 AT 2:00 pm

PART FINDER FRIDAY – PCF2129 – High precision real time clock with built in oscillator

Pt 10904

After our recent RTC part finder post, Adafruit friend Kevin Townsend wrote in to tell us about a accurate RTC that he’s been using, the PCF2129. Its half the cost of the DS3231 – but only runs up to 4.2V logic instead of 5V. Still, for all you using 3.3V logic or lipo’s its a great deal and a good replacement for the DS3231 when you need high accuracy temperature compensation timekeeping!


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  1. Can you please tell me why a company would put a chip into a 20 pin package when it only has 14 connected pins? Is it because the chip die is too big for a 14 pin DIP package? Thank you.

  2. @Marty- My guess is that most of those NC pins are used to calibrate the device at the factory. Once calibrated, they need to remain unconnected to prevent any sort of re-calibration. Because this needs to be a rather precision device, the chips would need to be calibrated on an individual basis.

  3. Because the crystal inside is fairly long … most 32.768KHz crystals are fairly longish, otherwise they’d be more than happy to save a few pennies in manufacturing on a smaller package.

  4. Thank you very much for your answers gentlemen. I appreciate them. Have a great weekend!

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