EM3027
4.4
Crystal Calibration
In order to compensate temperature dependency of the
used crystal, correct values of XtalOffset, Qcoef and TO
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:
ferr = -c1(T – c2)2 + c3 or fO = aT2 + bT + c.
6) Then real values of the searched
parameters can be obtained from the
following relations:
1) Supply the chip from VCC pin.
2) Set FD0 = FD1 = ‘0’. Set CLKOE pin to ’1’.
This provides the uncompensated
frequency signal from the crystal oscillator
directly on pin CLKOUT.
Qcoefreal = c1 = -a,
T0_real = c2 = -b/(2a),
XtalOffsetreal = c3 = c – b2/(4a).
3) Measure output frequency fO 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*Qcoefreal),
T0 = T0_real - 4,
4) Compute frequency deviation ferr in ppm of
output frequency fo from the ideal (target)
frequency fL = 32.768Hz in all measured
points as follows:
XtalOffset = 1.05*XtalOffsetreal.
ferr = 106(fo-fL)/fL .
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Copyright 2013, EM Microelectronic-Marin SA
3027-DS.doc, Version 8.0, 25-Jan-13
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