C8051F970-A-GM Datasheet, PDF (245/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
24.3.3. External Capacitor Mode
If a capacitor is used as the external oscillator, the circuit should be configured as shown in Figure 24.1, 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 MHz
R = pull-up resistor value in k
V_DD = power supply voltage in Volts
C = 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 = _C------^K_----^F-₃---_.--₀--
Since a frequency of roughly 150 kHz is desired, select the K Factor from Table 24.2 as KF = 22:
0.150 MHz = _C------_---²--₃--²-_.--₀-----_V---
C = ₀----_.-₁----₅---₀-----_M-----^2H---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.
24.3.4. 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.
246
Rev 1.0