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The 32.768 kHz quartz crystal was developed at the end of the 1960s for keeping time in wristwatches. This cheap-to-manufacture quartz resonator closely approximates the period of a balance-wheel from a mechanical clockwork and its frequency can be easily tuned down to 1 Hz by an IC. From this project, a 32.768 kHz quartz tuning fork resonator was born that was at first delivered mainly in round, metallic housings, and now also comes in rectangular, miniaturized SMD ceramic housings. During the development of the 32.768 kHz quartz crystal, it was discovered that a watch worn on the wrist experiences an average inherent warming of approximately 25 °C. Reading through the 32.768 kHz quartz crystal data sheets, one will notice a turning point in the frequency deviation in the range of 25 °C with a ±5 °C margin of fluctuation. Unlike a 1 MHz quartz crystal, which has a sine-shaped temperature characteristic, the 32.768 kHz quartz crystal has a temperature characteristic resembling an upside-down parabola. If a quartz crystal is operated outside -40 or 85 °C, then the frequency shift becomes at least -160 ppm, which equates to a time deviation of at least 428 seconds per month. At -20/+70 °C, it is at least -80 ppm, equating to a time deviation of at least 214 seconds per month.
Influences on frequency precision
In addition to the temperature characteristic, the time accuracy of an application is influenced by the pulling sensitivity (in ppm/pF) of the 32.768 kHz quartz crystal used. If the oscillator stage of the RTC used drifts – some have a delta of 20 pF – then this adds approximately another 7 to 10 ppm/pF frequency shift, depending on the load capacity of the 32.768 kHz quartz crystal. That means another approximately 140 to 200 ppm frequency shift in addition to the temperature drift. Furthermore, the aging, on average ±3 ppm/year, must be taken into account when calculating the timing precision.
Some IC RTC manufacturers recommend trimming the frequency using trim capacitors – which is an enormous effort, since the 32.768 kHz quartz crystal has to be trimmed to every individual RTC on the PCB.
Trimming will not get you very far
Trimming can only correct the frequency tolerance at 25 °C. Even if the frequency precision of the quartz crystal is trimmed by circuitry at 25 °C, the temperature drift and aging still remain, and have a very negative influence on the time accuracy of the application. As a rule of thumb, one can assume a 1 ppm frequency shift equates to about a 3-second time error per month. With a trimmed RTC in the working temperature range of -40/+85 °C, the time error would still be 428 seconds/month; if one also factors in the aging over 10 years as a linear function, this adds another 30 ppm = 90 seconds time error/month. Seen in terms of the characteristics of a 32.768 kHz quartz crystal, this quartz crystal is wholly unsuitable for yielding accurate RTC timing.
Compact RTC IC with DTCXO function
To be able to achieve very precise RTC timing, the clocking specialist CONTOC from Landsberg offers the RTC1010, with integrated quartz crystal and temperature-compensated oscillator (DTCXO). In the RTC1010, the behaviour described above for the 32.768 kHz quartz crystal is compensated for by a digital compensation network, by which highly precise RTC timing can be achieved. Over -40/+85 °C, the temperature stability of the RTC1010 is ±5 ppm, which allows a highly accurate time offset of 15 seconds absolute above the temperature. That makes the RTC1010 from CONTOC an extremely precise application timer for highest precision timestamping. The RTC1010 furthermore features a 400 kHz I²C Bus interface and an adjustable frequency output, which provides highly precise output frequencies (CMOS) with a choice of 32.768 kHz, 1.024 kHz or 1 Hz, which can be used to clock PLLs or to generate further system frequencies.
Extremely low power draw
The RTC1010, in its compact 14-pin plastic housing, can be operated over a supply voltage range of 1.6 to approx. 5.5 V. Typical applications use 3.0 V. At a supply voltage of +3.0 V, the power consumption of the RTC1010 is less than 1.8 µA. The RTC is therefore also suitable for battery-powered applications. In addition to the RTC IC, CONTOC has a wide product range of 32.768 kHz quartz crystals with frequency tolerances of ±10 ppm at 25 °C. The range also includes quartz resonators for other frequency ranges, oscillators and ceramic resonators. The catalogue of services is rounded off with circuit development, analysis and simulation, matching tests and much more.
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