AN663 データシートの表示（PDF） - Silicon Laboratories

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AN663

PRECISION32 MCU FAMILY CLOCKING OPTIONS

Silicon Laboratories 

AN663

2.4.2. CMOS Mode

When operating in CMOS mode, the external oscillator module supports CMOS oscillators with output frequencies

ranging from 10 kHz to 50 MHz. The external CMOS clock should be connected directly to the XTAL2 pin

configured in digital input mode. The XTAL1 pin should be left floating.

The CMOS oscillator mode is available with a divide by 2 stage, ensuring the clock has a 50% duty cycle.

2.4.3. RC Mode

The external oscillator module supports output frequencies up to 1.6 MHz when operating in RC mode. The RC

components should be connected to XTAL2 configured for analog mode, while XTAL1 should be left floating.

Figure 6 shows the external RC oscillator hardware configuration.

VIO

XTAL1

R

SiM3xxxx Device

EXTOSCn

Module

XTAL2

C

Figure 6. External RC Oscillator Configuration

The capacitor used in the RC network should have a value no greater than 100 pF, and the resistor should be no

smaller than 10 k. For very small capacitors, the parasitic capacitance in the PCB layout may dominate the total

capacitance. The oscillation frequency can be determined by Equation 3, where f is the frequency in MHz, R is the

pull-up resistor value in k., and C is the capacitor value in the XTAL2 pin in pF.

f

=

1----.--2---3---------1----0---3-

RC

Equation 3. RC Oscillation Frequency

The RC Oscillator mode is only available with a divide by 2 stage, which ensures that the clock derived from the

external oscillator has a duty cycle of 50%. The equation for the EXTOSCn output frequency is shown in

Equation 4.

fOUT

=

-f-

2

=

1----.-2----3---------1----0---3-

2RC

Equation 4. EXTOSCn Output Frequency in RC Mode

Rev. 0.1

7

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