Part # Application Note General Purpose Products M48T02 M48T08 M48T08Y M48T12 M48T128Y M48T129V M48T129Y M48T18 M48T35 M48T35AV M48T35Y datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: TIMEKEEPER calibration


Part Details:

AN934 APPLICATION NOTE TIMEKEEPER® Calibration INTRODUCTIONThe term "quartz accurate" has become a familiar phrase used to describe the accuracy of many timekeeping functions. Quartz oscillators provide an accuracy far superior to other conventional oscillator de-signs, but they are not perfect. Quartz crystals are sensitive to temperature variations. Figure 1 shows therelationship between accuracy (acc), temperature (T), and curvature (K) for the 32,768Hz crystal used onSTMicroelectronics TIMEKEEPER® products. The curves follow this general formula: a cc = K (T ­ T )2 × O where TO = 25°C ± 5°C and K = ­0.036 ppm/°C2 ± 0.006 ppm/°C2.The clocks used in most applications require a high degree of accuracy, and there are several factors in-volved in achieving this accuracy. Typically most crystals are compensated by adjusting the load capaci-tance of the oscillator. This method, while effective, has several disadvantages: 1. It requires external components (trim capacitors); and 2. it can increase oscillator current (an important factor in battery-supported applications). STMicroelectronics replaced this crude analog method with a digital calibration feature. This method givesthe user software control over the calibration procedure which makes it user friendly. Figure 1. Typical Crystal Accuracy plotted against Temperature (and against Different Values of K) 50 +35 ppm ­40 ­30 ­20 ­10 10 20 30 40 50 60 70 80 90 0 Temperature (°C) ­35 ppm ­50 ­100 Minimum K at 25°C Typical K at 25°C ­150 Maximum K at 25°C ­200 Accuracy (ppm) AI02498 November 2004 1/12 AN934 - APPLICATION NOTE TABLE OF CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 1. Typical Crystal Accuracy plotted against Temperature (and against Different Values of K)1 METHODOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Figure 2. Oscillator Divider Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 3. Clock Splitting and Clock Blanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 4. Control Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Table 1. Calibration Table: Compensation Values in seconds per month (30 days) and in ppm . . 5 CALCULATING THE NEEDED AMOUNT OF CALIBRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 CALCULATING CALIBRATION OVER A TEMPERATURE RANGE . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 5. Crystal Accuracy over a Temperature Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 CALCULATING THE CALIBRATION FOR MULTIPLE OPERATING TEMPERATURES . . . . . . . . . . 8


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Application Note General Purpose Products M48T02 M48T08 M48T08Y M48T12 M48T128Y M48T129V M48T129Y M48T18 M48T35 M48T35AV M48T35Y.pdf Datasheet