AN2971

Application theory

Doc ID 15627 Rev 1

5/12

2

Application theory

As shown in Figure 5, the typical temperature characteristics of the 32 KHz crystal are

described by a parabola. Away from the center point, at temperatures above and below

25°C, the crystal tends to run at lower frequencies. A table can be derived from the curve

from which the user can look up the approximate drift (in parts per million or PPM) of the

clock for a given temperature. Thus, if the application can measure the temperature, it can

predict the drift of the real-time clock and compensate accordingly by adjusting the RTC's

calibration circuit.

The M41T83 and M41T93 employ analog calibration circuits which adjust the frequency of

the oscillator by adding or removing capacitance. The non-linear relationship between the

load capacitance and frequency is depicted in the curve of Figure 7.

Note that, since the curve is non-linear, the incremental change in PPM of the oscillator

varies with the position on the curve. That is, a small change in capacitance does not have

the same effect on PPM at one point on the curve as at another. Therefore, in order to

determine how much incremental capacitance to add or remove to achieve a desired PPM

shift, the user must first determine where on the curve the device is operating.

In the cases of the M41T83 and M41T93, in the embedded crystal package, a factory-

calibration value is programmed into a permanent (one-time programmable) register which

can be read by the application. This provides the starting point for the calibration procedure.

It indicates where on the curve the device is operating at room temperature.

In summary, Figure 5 (tabularized in Figure 6) tells the user the necessary PPM of

calibration required for a given temperature. In turn, Figure 7 and Figure 8 allow the user to

determine the load capacitance setting necessary to achieve the desired PPM shift.