71M6521DE Datasheet, PDF (64/107 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/DH/FE Data Sheet
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
0
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.
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 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).
f
=
f _nom
⋅
¹

+
a
10⁶
+
T
b
10⁶
+
T
2
c
10⁶



When applying the inverted coefficients, a curve (see Figure 33) will result that effectively neutralizes the original
crystal characteristics.
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