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FM3808DK Datasheet(PDF) 8 Page - Ramtron International Corporation |
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FM3808DK Datasheet(HTML) 8 Page - Ramtron International Corporation |
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8 / 27 page  N O T R EC O M M EN D ED FO R N EW D ES IG N S La st Ti m e B uy : S ept . 200 6 N o Di re ct Re pl ac em en t Al te rn at e So lu tio n: FM 18 08 +F M 40 05 FM3808 Rev. 1.3 (EOL) Feb. 2006 Page 8 of 27 Calibration The RTC enters calibration mode when the CAL bit in register 7FF0h is set to 1. Interrupts are disabled in CAL mode. The RTC is calibrated by applying a digital correction to the RTC logic block. In CAL mode, the INT pin is driven with a 512.00 Hz nominal square wave. Based on the measured deviation from this frequency, a correction value must be written by the user into the calibration register 7FF8h. See Table 2 for the correction codes. 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, whereas 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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