71M6521FE-IMSLASHF datasheet(64/107 Pages) MAXIM

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71M6521DE/DH/FE Data Sheet

Page: 64 of 107

Rev 2

Temperature Compensation and Mains Frequency Stabilization for the RTC

The flexibility provided by the MPU allows for compensation of the RTC using the substrate temperature. To achieve

this, the crystal has to be characterized over temperature and the three coefficients Y_CAL, Y_CALC, and Y_CAL_C2

have to be calculated. Provided the IC substrate temperatures tracks the crystal temperature the coefficients can be

used in the MPU firmware to trigger occasional corrections of the RTC seconds count, using the RTC_DEC_SEC or

RTC_INC_SEC registers in I/O RAM.

Example: Let us assume a crystal characterized by the measurements shown in Table 63:

Deviation from

Nominal

Temperature [°C]

Measured

Frequency [Hz]

Deviation from

Nominal

Frequency [PPM]

+50

32767.98

-0.61

+25

32768.28

8.545

32768.38

11.597

-25

32768.08

2.441

-50

32767.58

-12.817

Table 63: Frequency over Temperature

The values show that even at nominal temperature (the temperature at which the chip was calibrated for energy), the

deviation from the ideal crystal frequency is 11.6 PPM, resulting in about one second inaccuracy per day, i.e. more

than some standards allow. As Figure 32 shows, even a constant compensation would not bring much improvement,

since the temperature characteristics of the crystal are a mix of constant, linear, and quadratic effects.

Figure 32: Crystal Frequency over Temperature

One method to correct the temperature characteristics of the crystal is to obtain coefficients from the curve in Figure

32 by curve-fitting the PPM deviations. A fairly close curve fit is achieved with the coefficients a = 10.89, b = 0.122,

and c = –0.00714 (see Figure 33).













⋅

nom

When applying the inverted coefficients, a curve (see Figure 33) will result that effectively neutralizes the original

crystal characteristics.

32767.5

32767.6

32767.7

32767.8

32767.9

32768

32768.1

32768.2

32768.3

32768.4

32768.5

-50

-25

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