EM3027
4.4
Crystal Calibration
In order to compensate temperature dependency of the
used crystal, correct values of XtalOffset, Qcoef and T
O
parameters shall be stored in EEPROM Control Page.
User is advised to follow these steps in order to compute
the parameters in a correct way:
5)
Find a quadratic regression of the
measured dependency in form:
f
err
= -c
1
(T – c
2
) + c
3
or f
O
= aT + bT + c.
2
2
6)
1)
Supply the chip from V
CC
pin.
Set FD0 = FD1 = ‘0’. Set CLKOE pin to ’1’.
This provides the uncompensated
frequency signal from the crystal oscillator
directly on pin CLKOUT.
Then real values of the searched
parameters can be obtained from the
following relations:
Qcoef
real
= c
1
= -a,
T
0_real
= c
2
= -b/(2a),
XtalOffset
real
= c
3
= c – b /(4a).
2
2)
3)
Measure output frequency f
O
at different
temperatures (at least five measurements
in equidistant points in the whole desired
temperature range are recommended).
Please note that quartz crystal needs few
minutes to stabilise its frequency at a given
temperature.
7)
The values to be stored in EEPROM
Control Page have to be corrected in the
following way:
Qcoef = 4096*(1.05*Qcoef
real
),
T
0
= T
0_real
- 4,
4)
Compute frequency deviation f
err
in ppm of
output frequency f
o
from the ideal (target)
frequency f
L
= 32.768Hz in all measured
points as follows:
6
f
err
= 10 (f
o
-f
L
)/f
L
.
XtalOffset = 1.05*XtalOffset
real
.
Copyright
2013, EM Microelectronic-Marin SA
3027-DS.doc, Version 8.0, 25-Jan-13
8
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EMMICRO [ EM MICROELECTRONIC - MARIN SA ]
