71M6521DE/71M6521FE
Energy Meter IC
DATASHEET
JANUARY 2008
32768.5
32768.4
32768.3
32768.2
32768.1
32768
32767.9
32767.8
32767.7
32767.6
32767.5
crystal
curve fit
inverse curve
-50
-25
0
25
50
Figure 31: 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 form:
Y _ CAL
Y _ CALC
Y _ CALC2
CORRECTION(ppm) =
+T ⋅
+T 2 ⋅
10
100
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):
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.
Connecting 5V Devices
All digital input pins of the 71M6521DE/FE are compatible with external 5V devices. I/O pins configured as inputs do not
require current-limiting resistors when they are connected to external 5V devices.
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