AN2678

Digital calibration

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Depending on whether this timer was also used to perform the initial analog calibration, an

adjustment may need to made to this number . Once the adjusted number is determined,

the error in ppm is calculated and the appropriate offset from Table 2 is selected and

programmed into the digital calibration register.

Example 1: A frequency counter was used during analog calibration, and the subsequent

period measured and calculated using the microprocessor’s timer was 0.0019531441

seconds. Ideally, this number would be 0.0019531250 (= 1/512). Thus, the microprocessor’s

timer is about 10 ppm fast, and the earlier number, 0.0019531441, will be used in place of

1/512 in Equation 1. Given a current calculated period of 0.0019531536, this is inserted into

the opposite amount. In Table 2, the positive value nearest this is +4 ppm, so the digital

calibration register would get would get DCS = 1 and DC4:DC0 = 00001.

Example 2: The microprocessor’s timer was used during analog calibration. As in example

1, let the calculated period be 0.0019531536 seconds. Inserting this into Equation 1 (and

retaining the 1/512), we get –14.65 ppm. The nearest opposite value is +16 ppm. For this,

DCS = 1 and DC4:DC0 = 00100.

With digital calibration, the adjustments are made open-loop. In the case of the analog

calibration adjustments, the frequency shift of the oscillator can be seen in the 512 Hz test

signal, but with the digital calibration, the effects are spread out over time and are not

immediately observable upon making changes. Furthermore, with digital calibration, no

iteration is required. Once the frequency error is known, an appropriate value is

programmed into the part and no further adjustments are made for while. The key idea is

that adjusting the digital calibration requires one measurement followed by one adjustment;

no looping is required.

This digital calibration procedure is repeated often enough to prevent the RTC from drifting

too far, but no more often than every 16 minutes, the interval at which the RTC digital

calibration algorithm updates. This can be reduced to 8 minutes when positive calibration

values are being used. As the ambient temperature changes, the oscillator will drift, and the

RTC oscillator will need adjustment. This can be done periodically by scheduling

calibrations, or, if a temperature sensor is available to the microprocessor, by monitoring for

changes in the temperature and adjusting the RTC when they occur. Or, a combination of

both might be utilized. For example, scheduled RTC calibrations might occur every hour,

and unscheduled ones might be run whenever a 2 degree temperature shift is detected.

As long as the microprocessor periodically adjusts the RTC, its timekeeping accuracy will be

optimized, and timekeeping errors minimized.