78M6612-IMR/F Maxim Integrated Products, 78M6612-IMR/F Datasheet - Page 65

• Manufacturer Part Number: 78M6612-IMR/F
• Description: IC POWER MEASUREMENT AC 68-QFN
• Manufacturer: Maxim Integrated Products

Specifications of 78M6612-IMR/F

• Mounting Style: SMD/SMT
• Package / Case: QFN-68
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

DS_6612_001
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
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
compensation would not bring much improvement, since the temperature characteristics of the crystal
are a mix of constant, linear, and quadratic effects.
One method to correct the temperature characteristics of the crystal is to obtain coefficients from the
curve in
coefficients a = 10.89, b = 0.122, and c = –0.00714 (see
When applying the inverted coefficients, a curve (see Figure 30) will result that effectively neutralizes the
original crystal characteristics.
Rev. 1.2
Figure 29
by curve-fitting the PPM deviations. A fairly close curve fit is achieved with the
Temperature [°C]
Deviation from
32768.5
32768.4
32768.3
32768.2
32768.1
32767.9
32767.8
32767.7
32767.6
32767.5
Figure 29: Crystal Frequency over Temperature
32768
Nominal
Table 45: Frequency over Temperature
+50
+25
-50
-25
-50
0
f
=
-25
f
nom
⋅
Frequency
Measured



32767.98
32768.28
32768.38
32768.08
32767.58
1
+
[Hz]
10
a
0
6
+
Figure
T
10
Figure 29
b
6
30).
+
Deviation from
25
T
Frequency
2
Nominal
-12.817
10
11.597
[PPM]
8.545
2.441
-0.61
c
6
shows, even a constant



50
Table
78M6612 Data Sheet
45.
65