External Capacitor Example
If a capacitor is used as the external oscillator, the circuit should be configured as shown in . 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 C mode.
XTAL1
XTAL2
Figure 8.8. External Capacitor Oscillator Configuration
The capacitor should be no greater than 100 pF; however, for very small capacitors, the total capacitance may be dominated by parasit-
ic capacitance in the PCB layout. The oscillation frequency and the required XFCN field value determined by the following equation,
where f is the frequency in MHz, C is the capacitor value on XTAL2 in pF, and V
DD
is the power supply voltage in Volts:
f
=
KF
C
×
V
DD
Figure 8.9. C Mode Oscillator Frequency
For example, assume VDD = 3.0 V and f = 150 kHz. Since a frequency of roughly 150 kHz is desired, select the K Factor from as KF =
22:
f
=
KF
C
×
V
DD
0.150 MHz =
22
C
× 3.0
C
=
22
0.150 MHz × 3.0
C
= 48.8 pF
Figure 8.10. C Mode Oscillator Example
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.
EFM8SB2 Reference Manual
Clocking and Oscillators
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