78M6612-IMSLASHFSLASHP datasheet(65/111 Pages) TERIDIAN

65 / 111 page

DS_6612_001

78M6612 Data Sheet

Rev. 1.2

3.4

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 45.

Table 45: Frequency over Temperature

Deviation from

Nominal

Temperature [°C]

Measured

Frequency

[Hz]

+50

Deviation from

Nominal

Frequency

[PPM]

32767.98

-0.61

+25

32768.28

8.545

32768.38

11.597

-25

32768.08

2.441

-50

32767.58

-12.817

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 29 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.

32767.5

32767.6

32767.7

32767.8

32767.9

32768

32768.1

32768.2

32768.3

32768.4

32768.5

-50

-25

Figure 29: Crystal Frequency over Temperature

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

curve in Figure 29 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 30).













⋅

nom

When applying the inverted coefficients, a curve (see Figure 30) will result that effectively neutralizes the

original crystal characteristics.

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