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CBC34803-M5C Datasheet(PDF) 4 Page - Cymbet Corporation |
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CBC34803-M5C Datasheet(HTML) 4 Page - Cymbet Corporation |
4 / 7 page AN-1058: Using CBC348xx EnerChip RTC in High Accuracy Applications ©2014 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com Doc AN-72-1058 Rev B Page 4 of 7 Thermal Use Case of Final System Different inaccuracies are present in each of the three modes of EnerChip RTC timing operation. For example, the crystal oscillator is much more accurate over temperature than is the RC oscillator by itself however more power is required. The Autocalibration RC Oscillator mode is a hybrid that provides the best combination of crystal accuracy and low power but adds some inaccuracies that may or may not affect the final system operation depending on the thermal use case. The system will be subjected to different temperature extremes for different periods of time and the amount of time the final product spends at each temperature extreme contributes a different amount of drift. It would be incorrect to rate the system’s overall accuracy at the accuracy associated with the worst temperature extreme if most of the time the product sits at 25°C, where the drift is very low. The crystal oscillator is capable of around 2-3 ppm accuracy after initial calibration to compensate for initial crystal frequency and parasitic capacitances and thermal compensation for the crystal. A procedure for calibrating the initial frequency including parasitic is given later in this paper. If the system experienced an additional 20ppm error from temperature variation for only one day per year and operated very near 25°C for the rest of the time it would be wrong to specify the accuracy at 20ppm. The real accuracy would be much closer to the 2-3ppm it was initially calibrated to. Initial Frequency Trimming The CBC348xx devices contain registers to compensate for the initial frequency inaccuracy and parasitic loads of the final circuit board layout and build. These registers can be loaded at run time from the product’s microcomputer and the values are easily determined in approximately one second or less at test time. Trimming the initial frequency must be done with a well-calibrated timing source to assure the best operation. The algorithm for trimming will be discussed later in this paper. Crystal Aging All crystals age over time. The bulk of the aging happens in the first few months of operation. Aging can be caused by matter falling off the crystal or matter falling onto the crystal. Crystal manufacturers specify aging in ppm for the first year and the numbers typically vary from ±2ppm to± 4ppm. Because the CBC348xx has no way of knowing how the crystal will age it is up to the application software to compensate for this. One could determine the difference in time when the time is re-set and calculate the amount to modify the calibration registers. Remember that aging mostly happens in the first few months of operation and so is a diminishing factor in the overall system accuracy over time. |
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