Most RTCs are inaccurate to this degree,...

Hallo Jacob,

hmm, is 174ppm really normal?

If one has a look at datasheet of PCF8583, which is a widely used RTC containing I2C interface, tolerance of chip is specified to be +-5ppm on average. If one adds the +-20ppm max deviation of a good quality 32.768kHz quartz (AEL) and temperature deviation of -0.04ppm/°C, then we result with 5ppm + 20ppm + 0.04ppm/°C x 20°C = 26ppm. This is much less than 174ppm. And if one uses methode of calibration, either mechanically or by software, then even much less deviation is possible. Datasheet of PCF8583 tells, that overall average deviation of +-5 minutes per year can be achieved. +-5 minutes per year is 10ppm, or 0.9sec per day.

Assuming that Azhar is using a RTC with similar precision, then even without calibration something about 25ppm should be achievable. But if he gets 174ppm, then something is wrong with his circuit. Either he uses very poor crystal with much higher frequency tolerance, or he omits certain burden capacitance, or something else. That was my consideration.

May be, Azhar can come back and tell us, what RTC he is using?


Here the according datasheets:

http://www.aelcrystals.co.uk/pdf-files...lPagef.pdf
http://www.amresearch.com/datasheets/pcf8583.pdf

Regards,
Kai