AN663 データシートの表示（PDF） - Silicon Laboratories
部品番号
コンポーネント説明
メーカー
AN663 Datasheet PDF : 12 Pages
| |||
AN663
2.4.1. Crystal Mode
When operating in crystal mode, the external oscillator module supports crystals ranging from 10 kHz to 25 MHz.
The module requires a 10 Mresistor across the crystal pins and two loading capacitors between the crystal pins
and ground. Both of the crystal pins must be configured for analog input mode. Figure 5 shows the external crystal
oscillator hardware configuration.
SiM3xxxx Device
C
XTAL1
10 M
EXTOSCn
Module
XTAL2
C
Figure 5. External Crystal Oscillator Configuration
The capacitors 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 recommended load capacitance depends upon the crystal and the manufacturer. Refer to the crystal data sheet
when completing these calculations.
Equation 1 describes the equation for determining the load capacitance for the two capacitors. The CA and CB
values are the capacitors connected to the crystal leads. The CS value is the total stray capacitance of the PCB,
which is typically between 2 pF and 5 pF per pin for a typical layout where the crystal is as close as possible to the
pins.
CL
=
-C----A---------C-----B--
CA + CB
+
CS
Equation 1. Crystal Load Capacitors
If CA and CB are the same (C), the resulting equation is shown in Equation 2.
CL
=
C---
2
+
CS
Equation 2. Simplified Crystal Load Capacitors
For example, using Equation 2 with a 32.768 kHz tuning-fork crystal with a recommended load capacitance of
12.5 pF placed as close to the pins as possible (3 pF per pin) results in crystal load capacitors of 13 pF each.
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 that could introduce noise or interference.
6
Rev. 0.1
Share Link: 