C8051F99X_10 Datasheet, PDF (190/322 Pages) Silicon Laboratories – Ultra Low Power, 8-2 kB Flash, Capacitive Sensing MCU

C8051F99x-C8051F98x
19.3.3. External Capacitor Mode
If a capacitor is used as the external oscillator, the circuit should be configured as shown in Figure 19.1,
Option 3. The capacitor should be added to XTAL2, and XTAL2 should be configured for analog I/O with
the digital output drivers disabled. XTAL1 is not affected in RC mode.
The capacitor should be no greater than 100 pF; however, for very small capacitors, the total capacitance
may be dominated by parasitic capacitance in the PCB layout. The oscillation frequency and the required
External Oscillator Frequency Control value (XFCN) in the OSCXCN Register can be determined by the
following equation:
f = -------K-----F---------
C  V_DD
where
f = frequency of clock in MHzR = pull-up resistor value in k
V_DD = power supply voltage in VoltsC = capacitor value on the XTAL2 pin in pF
Below is an example of selecting the capacitor and finding the frequency of oscillation Assume V_DD = 3.0 V
and f = 150 kHz:
f = -------K-----F---------
C  V_DD
0.150 MHz = ------K----F-------
C  3.0
Since a frequency of roughly 150 kHz is desired, select the K Factor from Table 19.2 as KF = 22:
0.150 MHz = _{C------}---2--3-2--.-0-----V---
C = _{0---.--1---5---0-----M-----H2---2-z----}-----3---.--0----V---
C = 48.8 pF
Therefore, the XFCN value to use in this example is 011 and C is approximately 50 pF.
The recommended startup procedure for C mode is the same as RC mode.
19.3.4. External CMOS Clock Mode
If an external CMOS clock is used as the external oscillator, the clock should be directly routed into XTAL2.
The XTAL2 pin should be configured as a digital input. XTAL1 is not used in external CMOS clock mode.
The external oscillator valid detector will always return zero when the external oscillator is configured to
External CMOS Clock mode.
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