NXP Semiconductors UM10301 PCF2123 - Manual (Page 12)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 12 of 52
capacitances. The external components of the oscillator have to be chosen such that the
actual value of C
L
matches the specified value of C
L
. If there is mismatch the crystal will
not run exactly at its specified frequency resulting in the clock running slow or fast.
The crystal manufacturer can manufacture crystals for any load capacitance, but in
practice some standard values are used. For use in real-time clocks you may find
crystals specified for load capacitances of 7 pF, 9 pF and 12.5 pF with 12.5 pF the most
common value.
(1) Frequency on the left scale and the equivalent deviation from the nominal frequency in ppm
on the right scale
Fig 6. Graph of oscillator frequency as function of load capacitance C
L
Fig 6 depicts the influence of the load capacitance applied to the crystal on the oscillator
frequency. The lower curve represents a crystal with a specified C
L
of 7 pF, the upper
curve represents a crystal with a specified C
L
of 12 pF. From this graph it is obvious that
the 7 pF crystal is more sensitive to deviations from the specified C
L
. If the applied C
L
is 1
pF lower than specified, the frequency deviation will be 18 ppm, whereas the 12.5 pF
crystal will only show a frequency deviation of 6 ppm if the applied C
L
is 1 pF below the
specified value. This is not surprising since the same absolute change in load
capacitance is a larger relative change if the load capacitance is smaller. A lower load
capacitance however will result in lower power consumption and in cases where this is
an important requirement a crystal with lower required C
L
could be selected.