C8051F52X_12 Datasheet, PDF (140/221 Pages) Silicon Laboratories – 8/4/2 kB ISP Flash MCU Family

C8051F52x/F53x
Note: The load capacitance depends upon the crystal and the manufacturer. Please refer to the crystal
data sheet when completing these calculations.
The equation for determining the load capacitance for two capacitors is:
C_L
=
-^C----^A----^-----^C----^B-
C_A + C_B
+
C_S
Where:
C_A and C_B are the capacitors connected to the crystal leads.
C_S is the total stray capacitance of the PCB.
The stray capacitance for a typical layout where the crystal is as close as possible to the pins is 2–5 pF per
pin.
If C_A and C_B are the same (C), then the equation becomes:
C_L
=
C_---
2
+
C_S
For example, a tuning-fork crystal of 32 kHz with a recommended load capacitance of 12.5 pF should use
the configuration shown in Figure 14.1, Option 1. With a stray capacitance of 3 pF per pin (6 pF total), the
13 pF capacitors yield an equivalent capacitance of 12.5 pF across the crystal, as shown in Figure 14.2.
13 pF
XTAL1
32 kHz
10 M^
13 pF
XTAL2
Figure 14.2. 32 kHz External Crystal Example
Important Note on External Crystals: Crystal oscillator circuits are quite sensitive to PCB layout. The
crystal should be placed as close as possible to the XTAL pins on the device. The traces should be as
short as possible and shielded with ground plane from any other traces which could introduce noise or
interference.
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Rev. 1.4