DS_6612_001
78M6612 Data Sheet
3.4
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 45: Frequency over Temperature
Deviation from
Nominal
Temperature [°C]
+50
+25
0
-25
-50
Measured
Frequency
[Hz]
32767.98
32768.28
32768.38
32768.08
32767.58
Deviation from
Nominal
Frequency
[PPM]
-0.61
8.545
11.597
2.441
-12.817
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
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 29: Crystal Frequency over Temperature
One method to correct the temperature characteristics of the crystal is to obtain coefficients from the
curve in
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
a
b
c
f
=
f
nom
⋅
1
+
6
+
T
6
+
T
2 6
�½
10
10
10
When applying the inverted coefficients, a curve (see Figure 30) will result that effectively neutralizes the
original crystal characteristics.
Rev. 1.2
65
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
