71M6521DE Datasheet, PDF (65/107 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/DH/FE Data Sheet
32768.5
32768.4
32768.3
32768.2
32768.1
32768
32767.9
32767.8
crystal
32767.7
32767.6
curve fit
inverse curve
32767.5
-50
-25
0
25
50
Figure 33: Crystal Compensation
The MPU Demo Code supplied with the Teridian Demo Kits has a direct interface for these coefficients and it directly
controls the RTC_DEC_SEC or RTC_INC_SEC registers. The Demo Code uses the coefficients in the form:
CORRECTION ( ppm) = ^{Y _ CAL} + T ⋅ ^{Y _ CALC} + T ² ⋅ ^{Y _ CALC2}
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 5 V Devices
All digital input pins of the 71M6521DE/DH/FE are compatible with external 5 V devices. I/O pins configured as inputs
do not require current-limiting resistors when they are connected to external 5 V devices.
Rev 2
Page: 65 of 107