Application Note 1400 ISL12022 RTC Accuracy Optimization Calibration Procedure
Real Time Clock (RTC) devices contain an oscillator and normally require a crystal to operate with the required accuracy. Most common 32.768kHz crystal oscillators have a frequency tolerance of ±20ppm at +25°C, and a nonlinear temperature coefficient - Alpha - of -0.034ppm/°C². The temperature induced frequency drift is parabolic in nature, worsening as the crystal temperature deviates from the turnover temperature, referred to as the crystal’s T0, which is commonly +25°C.
The following example illustrates the effects of frequency tolerance and Alpha on the accuracy of an RTC.
A crystal oscillator has a frequency error of -15ppm at +25°C and an Alpha of -0.034ppm/°C². At +40°C, the error becomes -22.7ppm (-15ppm + [-0.034ppm/°C²*(40°C -25°C)²]). A -22.7ppm drift means the RTC runs 22.7μs slower in a second, which seems very small at first glance, but accumulates to many minutes in a year. A -22.7ppm drift means losing 2 seconds in a day, 14 seconds in a week, 59 seconds in a month, and 716 seconds (11.9 minutes) in a year. A user of most common RTCs has to live with this error, but the ISL12022 RTC, with an embedded temperature compensation function, solves this problem. This application note describes a simple trimming procedure that optimizes the accuracy of an ISL12022 RTC, and presents calibrated and uncalibrated results for several crystals.
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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April 19, 2013 |
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AN1400.1 |
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Application Note 1400 |
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284 KB |
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