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FM3808-70-T Datasheet(PDF) 8 Page - Ramtron International Corporation |
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FM3808-70-T Datasheet(HTML) 8 Page - Ramtron International Corporation |
8 / 28 page FM3808 Rev 1.1 May 2003 Page 8 of 28 32.768 kHz crystal Oscillator Clock Divider Update Logic 32 Hz 512 Hz W R Seconds 7 bits Minutes 7 bits Hours 6 bits Date 6 bits Months 5 bits Years 8 bits CF Days 3 bits User Interface Registers 1 Hz Figure 2. Real-Time Clock Core Block Diagram Calibration When the CAL bit in register 7FF0h is set to 1, the clock enters calibration mode. Interrupts are disabled in CAL mode. Calibration operates by applying a digital correction to the counter based on the frequency error. In CAL mode, the INT pin is driven with a 512 Hz nominal square wave. Any measured deviation from 512 Hz is converted into an error in ppm. This error corresponds to a correction value that must then be written by the user into the calibration register 7FF8h. The correction factors are listed in the Table 2. Positive ppm errors require a negative adjustment that removes pulses. Negative ppm errors require a positive correction that adds pulses. Positive ppm adjustments have the CALS bit set to 1, where as negative ppm adjustments have CALS = 0. After calibration, the clock will have a maximum error of ± 4.34 ppm or ± 0.19 minutes per month at the calibrated temperature. The calibration setting is nonvolatile and is stored in 7FF8h (bits D4-D0). This value only can be written when the CAL bit is set to a 1. To exit calibration mode, the user must clear the CAL bit to a 0. When the calibration mode is entered, the user can measure the frequency error on the INT pin. This error expressed in ppm translates directly into timekeeping error. An offsetting calibration adjustment corrects this error. However, the correction is applied by adding or removing pulses on a periodic basis. Therefore, the correction will not appear on the 512 Hz output. The calibration correction must be applied using the values shown in Table 2. The timekeeping accuracy can be verified by comparing the FM3808 time to a reference source. |
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