AN2678

Calibrating the M41T83 using analog calibration

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Calibrating the M41T83 using analog calibration

The analog calibration procedure is performed only once, at room temperature (25°C). It

made. This cycle is repeated until the error is minimized, that is, until the oscillator can be

fine tuned no further. At that point, the RTC's accuracy will be approximately equal to the

microprocessor's oscillator accuracy, which, depending on the quality of its crystal, may not

vary more than +/–5 ppm over a wide temperature range.

2.1

Analog calibration circuit

The analog calibration circuit utilizes an array of load capacitors internal to the RTC as

shown in the inset of Figure 4.

Values written to the analog calibration register add or remove load capacitance to slow

down or speed up the oscillator, respectively. The 32 KHz crystals are designed to see a

specific load capacitance. ST builds this into their real-time clocks; users do not need to add

any external capacitance to the crystals. Values programmed into the analog calibration

register adjust the amount of this built-in load capacitance seen by the crystal.

are 25 pF each. The adjustment range of the RTC is such that up to 18 pF can be

subtracted from each, or up to 9.75 pF can be added making the full range of each load

capacitor 7 to 34.75 pF.

The equivalent load capacitance is the series combination of the two, and is hence

nominally 12.5 pF with a range of 3.5 pF to 17.4 pF. (In other words, for identical capacitors,

the series equivalent value is ½ the individual values.)

The asymmetry in the adjustment range is due to the fact that the crystals tend to need

speeding up much more often than they need slowing down. This is evident upon

examining the watch crystal curve in Figure 1. Thus, much more capacitance can be

subtracted from nominal than can be added.