C8051F352 Datasheet, PDF (132/234 Pages) Silicon Laboratories – 8 k ISP Flash MCU Family

C8051F350/1/2/3
The capacitors shown in the external crystal configuration provide the load capacitance required by the
crystal for correct oscillation. These capacitors are “in series” as seen by the crystal and “in parallel” with
the stray capacitance of the XTAL1 and XTAL2 pins.
Note: The load capacitance depends upon the crystal and the manufacturer. Please refer to the crystal
datasheet when completing these calculations.
For example, a tuning-fork crystal of 32.768 kHz with a recommended load capacitance of 12.5 pF should
use the configuration shown in Figure 17.1, Option 1. The total value of the capacitors and the stray
capacitance of the XTAL pins should equal 25 pF. With a stray capacitance of 3 pF per pin, the 22 pF
capacitors yield an equivalent capacitance of 12.5 pF across the crystal, as shown in Figure 17.2.
22 pF
32.768 kHz
XTAL1
10 MΩ
22 pF
XTAL2
Figure 17.2. 32.768 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. 1.1