71M6511 Datasheet, PDF (78/95 Pages) Teridian Semiconductor Corporation – Single-Phase Energy Meter IC

71M6511/71M6511H
Single-Phase Energy Meter IC
DATA SHEET
AUGUST 2007
When applying the inverted coefficients, a curve (see Figure 32) will result that effectively neutralizes the original crystal
characteristics. The frequencies were calculated using the fit coefficients as follows:
f
=
f
nom
⋅
¹

+
a
10⁶
+
T
b
10⁶
+
T
2
c
10⁶


32768.5
32768.4
32768.3
32768.2
32768.1
32768
32767.9
32767.8
32767.7
32767.6
crystal
curve fit
inverse curve
32767.5
-50
-25
0
25
50
Figure 32: Crystal Compensation
The MPU Demo Code supplied with the TERIDIAN Demo Kits has a direct interface for these coefficients and it directly con-
trols the RTC_DEC_SEC or RTC_INC_SEC registers. The Demo Code uses the coefficients in the following form:
CORRECTION ( ppm)
=
Y
_ CAL
10
+T
⋅^Y
_ CALC
100
+T²
⋅
Y
_ CALC2
1000
Note that the coefficients are scaled by 10, 100, and 1000 to provide more resolution. For our example case, the coefficients
would then become (after rounding, since the Demo Code accepts only integers):
Y_CAL = 109, Y_CALC = 12, Y_CALC2 = 7
Alternatively, the mains frequency may be used to stabilize or check the function of the RTC. For this purpose, the CE provides
a count of the zero crossings detected for the selected line voltage in the MAIN_EDGE_X address. This count is equivalent to
twice the line frequency, and can be used to synchronize and/or correct the RTC.
External Temperature Compensation
In a production electricity meter, the 71M6511 or 71M6511H is not the only component contributing to temperature de-
pendency. In fact, a whole range of components (e.g. current transformers, resistor dividers, power sources, filter capacitors)
will exhibit slight or pronounced temperature effects. Since the output of the on-chip temperature sensor is accessible to the
MPU, temperature-compensation mechanisms with great flexibility, i.e. beyond the capabilities implemented in the CE, are
possible.
Temperature Measurement
Temperature measurement can be implemented with the following steps:
1) At a known temperature T_N, read the TEMP_RAW register of the CE and write the value into TEMP_NOM.
2) Read the TEMP_X register at the known temperature. The obtained value should be <±0.1°C.
3) The temperature T (in °C) at any environment can be obtained by reading TEMP_X and applying the following
formula:
T
=
T_N
+
TEMP _
10
X
Page: 78 of 95
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