CY28435
As mentioned previously, the capacitance on each side of the
crystal is in series with the crystal. This mean the total capac-
itance on each side of the crystal must be twice the specified
load capacitance (CL). While the capacitance on each side of
the crystal is in series with the crystal, trim capacitors
(Ce1,Ce2) should be calculated to provide equal capacitance
loading on both sides.
Use the following formulas to calculate the trim capacitor
values for Ce1 and Ce2.
Figure 2. Crystal Capacitive Clarification
Load Capacitance (each side)
Calculating Load Capacitors
Ce = 2 * CL – (Cs + Ci)
In addition to the standard external trim capacitors, trace
capacitance and pin capacitance must also be considered to
correctly calculate crystal loading. As mentioned previously,
the capacitance on each side of the crystal is in series with the
crystal. This means the total capacitance on each side of the
crystal must be twice the specified crystal load capacitance
(CL). While the capacitance on each side of the crystal is in
series with the crystal, trim capacitors (Ce1,Ce2) should be
calculated to provide equal capacitive loading on both sides.
Total Capacitance (as seen by the crystal)
1
CLe
=
1
Ce2 + Cs2 + Ci2
1
Ce1 + Cs1 + Ci1
(
)
+
CL....................................................Crystal load capacitance
CLe......................................... Actual loading seen by crystal
using standard value trim capacitors
Ce..................................................... External trim capacitors
Cs..............................................Stray capacitance (terraced)
Clock Chip
Ci ...........................................................Internal capacitance
(lead frame, bond wires etc.)
Ci2
Ci1
Pin
3 to 6p
X2
X1
Cs2
Cs1
Trace
2.8pF
XTAL
Ce1
Ce2
Trim
33pF
Figure 3. Crystal Loading Example
Rev 1.0,November 20, 2006
Page 11 of 22
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