AN3566| Application Note
Maxim/Dallas > App Notes > OSCILLATORS/DELAY LINES/TIMERS/COUNTERS TEMPERATURE SENSORS and THERMAL MANAGEMENT
REAL-TIME CLOCKS
Keywords: Accuracy, Timekeeping, Calibration, DS3231, Temperature Compensation, TCXO, Accurate RTC, 32.768 kHz
Jun 14, 2005
APPLICATION NOTE 3566
Timekeeping Accuracy, Automatic and Affordable
Electronic timekeeping has always lacked a high level of accuracy due to the inferior characteristics of the quartz crystal over temperature. Many different techniques have been applied to improve the accuracy provided by a 32.768kHz quartz crystal. This article describes a highly integrated device that provides unparalleled timekeeping accuracy at a price point comparable to an uncalibrated stand-alone real-time clock (RTC). This device will make current accuracy-improvement techniques obsolete, helping to make accurate timekeeping become the standard rather than a luxury. "You may delay, but time will not." - Benjamin Franklin If Benjamin Franklin had to use a quartz crystal and an RTC to maintain the time of day, he may have rethought his statement. The inaccuracy of the crystal over temperature usually makes time appear to delay (or, occasionally, to move quicker). An RTC with a 32.768kHz quartz tuning-fork crystal oscillator is the standard timekeeping reference for most electronic applications. The RTC
maintains the time and date by counting seconds, which requires a 1Hz clock signal derived from the 32.768kHz crystal oscillator. The current time and date information is stored in a set of registers, which is accessed through a communication interface.
The Problem
There is nothing inherently wrong with using an RTC for timekeeping. However, the time will only be as accurate as the reference used. Unfortunately, the typical 32.768kHz tuning-fork crystal does not provide much accuracy over a wide temperature range. Due to its parabolic nature over temperature (Figure 1), this accuracy is typically 20ppm at room temperature (+25 C). This is the equivalent of gaining or losing 1.7 seconds of time each day, or 10.34 minutes per year. As Figure 1 shows, accuracy decreases at more extreme high and low temperatures. The typical accuracy at these temperatures is much worse than 150ppm, which is the equivalent to losing almost 13.0 seconds of time each day, or over 1.3 hours per year.
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