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M48T59Y-70PC1TR Datasheet(PDF) 11 Page - STMicroelectronics |
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M48T59Y-70PC1TR Datasheet(HTML) 11 Page - STMicroelectronics |
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11 / 17 page  Stopping and Starting the Oscillator The oscillator may be stopped at any time. If the device is going to spend a significant amount of time on the shelf, the oscillator can be turned off to minimize current drain on the battery. The STOP bit is the MSB of the seconds register. Setting it to a ’1’ stops the oscillator. The M48T58/58Y is shipped from STMicroelectronics with the STOP bit set to a ’1’. When reset to a ’0’, the M48T58 oscillator starts within one second. Calibrating the Clock The M48T58/58Y is driven by a quartz controlled oscillator with a nominal frequency of 32,768 Hz. The devices are tested not to exceed 35 ppm (parts per million) oscillator frequency error at 25 °C, which equates to about ± 1.53 minutes per month. With the calibration bits properly set, the accuracy of each M48T58 improves to better than ±4 ppm at 25 °C. The oscillation rate of any crystal changes with temperature (see Figure 10). Most clock chips com- pensate for crystal frequency and temperature shift error with cumbersome trim capacitors. The M48T58/58Y design, however, employs periodic counter correction. The calibration circuit adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage, as shown in Figure 9. The number of times pulses are blanked (sub- AI02124 -80 -60 -100 -40 -20 0 20 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 ∆F = -0.038 (T - T 0) 2 ± 10% F ppm C 2 T 0 = 25 °C ppm °C Figure 10. Crystal Accuracy Across Temperature tracted, negative calibration) or split (added, posi- tive calibration) depends upon the value loaded into the five Calibration bits found in the Control Register. Adding counts speeds the clock up, sub- tracting counts slows the clock down. The Calibration byte occupies the five lower order bits (D4-D0) in the Control register (1FF8h). These bits can be set to represent any value between 0 and 31 in binary form. Bit D5 is a Sign bit; ’1’ indicates positive calibration, ’0’ indicates negative calibration. Calibration occurs within a 64 minute cycle. The first 62 minutes in the cycle may, once per minute, have one second either shortened by 128 or lengthened by 256 oscillator cycles. If a binary ’1’ is loaded into the register, only the first 2 minutes in the 64 minute cycle will be modified; if a binary 6 is loaded, the first 12 will be affected, and so on. Therefore, each calibration step has the effect of adding 512 or subtracting 256 oscillator cycles for every 125,829,120 actual oscillator cycles; that is +4.068 or -2.034 ppm of adjustment per calibration step in the calibration register. Assuming that the oscillator is in fact running at exactly 32,768 Hz, each of the 31 increments in the Calibration byte would represent +10.7 or - 5.35 seconds per month which corresponds to a total range of +5.5 or - 2.75 minutes per month. 11/17 M48T58, M48T58Y |
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