EM3027
4.3
Crystal Thermal Behaviour
The following formula expresses a compensation value
to be used during frequency correction.
2
COMP_val
½
Qcoef
(T
To)
XtalOffset
Qcoef
T
T
O
XtalOffset
COMP_val
-300
The frequency of the crystal is dependent on the
temperature concurring with the following diagram:
0
F
[ppm]
F
O
-100
-200
– Thermal quadratic coefficient [ppm/°C
2
]
– Actual temperature [°C]
– Turnover temperature [°C]
– Crystal offset at T
O
[ppm]
– Compensation value result [ppm]
-400
T
O
-100
T
O
-50
T
O
T
O
+50
T
O
+100
T [°C]
Temperature [°C]
Figure 1: Crystal thermal behaviour
T
O
– Turnover temperature [°C]
F
O
– Crystal frequency when T
O
[Hz]
The oscillator frequency is adjusted according to the
equation above by using coefficients located in the
EEPROM control page and the temperature.
The actual temperature can be obtained from the internal
thermometer or from Temp register updated externally by
an application.
The principle of the frequency compensation is based on
adding/removing of pulses.
Example 1:
Qcoef=0.035; T
O
=25; XtalOffset=–100
Example 2:
Qcoef=0.035; T
O
=25; XtalOffset=+100
400
600
[ppm]
300
[ppm ]
400
200
200
100
Tem perature
Temperature
0
-50
0
50
100
150
-50
0
0
-100
50
100
150
-200
-200
-400
Compensation Value
Crystal Error
-300
Compensation Value
Crystal Error
-400
-600
Copyright
2013, EM Microelectronic-Marin SA
3027-DS.doc, Version 8.0, 25-Jan-13
7
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EMMICRO [ EM MICROELECTRONIC - MARIN SA ]
