71M6521DE/71M6521FE
Energy Meter IC
DATASHEET
JANUARY 2008
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 61:
Deviation from
Nominal
Temperature [°C]
Deviation from
Nominal
Frequency [PPM]
Measured
Frequency [Hz]
+50
+25
0
32767.98
32768.28
32768.38
32768.08
32767.58
-0.61
8.545
11.597
2.441
-25
-50
-12.817
Table 61: 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 30 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.
32768.5
32768.4
32768.3
32768.2
32768.1
32768
32767.9
32767.8
32767.7
32767.6
32767.5
-50
-25
0
25
50
Figure 30: Crystal Frequency over Temperature
One method to correct the temperature characteristics of the crystal is to obtain coefficients from the curve in Figure 30 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 31).
a
106
b
106
c
106
⎧
⎩
⎫
⎬
⎭
f = fnom ⋅ 1+
+T
+T 2
⎨
When applying the inverted coefficients, a curve (see Figure 31) will result that effectively neutralizes the original crystal
characteristics. The frequencies were calculated using the fit coefficients as follows:
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