C8051F336/7/8/9
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 desired load capacitance depends upon the crystal and the manufacturer. Please refer to the
crystal data sheet 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 19.1, Option 1. The total value of the capacitors and the stray capac-
itance 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 19.2.
XTAL1
10MΩ
XTAL2
32.768 kHz
22pF*
22pF*
* Capacitor values depend on
crystal specifications
Figure 19.2. External 32.768 kHz Quartz Crystal Oscillator Connection Diagram
124
Rev. 0.2
SILABS [ SILICON LABORATORIES ]
