71M6511 Datasheet, PDF (77/95 Pages) Teridian Semiconductor Corporation – Single-Phase Energy Meter IC

71M6511/71M6511H
Single-Phase Energy Meter IC
DATA SHEET
AUGUST 2007
Temperature Compensation and Mains Frequency Stabilization for the RTC
The accuracy of the RTC depends on the stability of the external crystal. Crystals vary in terms of initial accuracy as well as in
terms of behavior over temperature. The flexibility provided by the MPU allows for compensation of the RTC using the sub-
strate 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.
It is not recommended to measure crystal frequency directly due to the error introduced by the measurement probes. A
practical method to measure the crystal frequency (when installed on the PCB with the 71M6511) is to have a DIO pin toggle
every second, based on the RTC interrupt, with all other interrupts disabled. When this signal is measured with a precision
timer, the crystal frequency can be obtained from the measured time period t (in µs):
f = 3276810^{6 µs}
t
Example: Let us assume a crystal characterized by the measurements shown in Table 62. 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.
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 62: Frequency over Temperature
As Figure 31 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 (in commercially available crystals, the constant and quadratic
effects are dominant).
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 31: Crystal Frequency over Temperature
The temperature characteristics of the crystal are obtained from the curve in Figure 31 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 32).
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