AN663 Datasheet, PDF (8/12 Pages) Silicon Laboratories – PRECISION32™ MCU FAMILY CLOCKING OPTIONS

AN663
2.4.4. C Mode
When operating in C mode, the external oscillator module supports frequencies up to 1.6 MHz. The capacitor
should connect directly to XTAL2 configured for analog mode. XTAL1 should be left floating. Figure 7 shows the
external C oscillator hardware configuration.
SiM3xxxx Device
XTAL1
XTAL2
C
EXTOSCn
Module
Figure 7. External C Oscillator Configuration
To determine the required module settings, select the capacitor to be used and find the frequency of oscillation
according to Equation 5, where f is the frequency of oscillation in MHz, C is the capacitor value in pF, V_DD is the
device power supply in Volts, and KF is the K Factor.
f
=
-------^K-----^F---------
C  V_DD
Equation 5. C Oscillation Frequency
The C 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 6.
f_OUT
=
-^f-
2
=
2^-----^-----C^--K----^-F----V^----_D^----_D^-
Equation 6. EXTOSCn Output Frequency in C Mode
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Rev. 0.1