C8051F50x-F51x
XTAL1
10M
XTAL2
32.768 kHz
22pF*
22pF*
* Capacitor values depend on
crystal specifications
Figure 19.3. External 32.768 kHz Quartz Crystal Oscillator Connection Diagram
19.4.2. External RC Example
If an RC network is used as an external oscillator source for the MCU, the circuit should be configured as
shown in Figure 19.1, Option 2. 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. To deter-
mine the required External Oscillator Frequency Control value (XFCN) in the OSCXCN Register, first
select the RC network value to produce the desired frequency of oscillation, according to Equation , where
f = the frequency of oscillation in MHz, C = the capacitor value in pF, and R = the pull-up resistor value in
k
.
f
=
1.23
10
R
C
Equation 19.1. RC Mode Oscillator Frequency
For example: If the frequency desired is 100 kHz, let R = 246 k
and C = 50 pF:
f = 1.23(10
3
)/RC = 1.23(10
3
)/[246 x 50] = 0.1 MHz = 100 kHz
Referring to the table in SFR Definition 19.6, the required XFCN setting is 010b.
19.4.3. External Capacitor Example
3
If a capacitor is used as an external oscillator for the MCU, the circuit should be configured as shown in
the total capacitance may be dominated by parasitic capacitance in the PCB layout. To determine the
required External Oscillator Frequency Control value (XFCN) in the OSCXCN Register, select the capaci-
tor to be used and find the frequency of oscillation according to Equation 19.2, where f = the frequency of
oscillation in MHz, C = the capacitor value in pF, and V
DD
= the MCU power supply in Volts.
Rev. 1.1
175
SILABS [ SILICON LABORATORIES ]
