C8051F70X_0910 Datasheet, PDF (163/290 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F70x/71x
10M
XTAL1
XTAL2
22pF*
32.768 kHz
22pF*
* Capacitor values depend on
crystal specifications
Figure 25.2. External 32.768 kHz Quartz Crystal Oscillator Connection Diagram
25.3.2. 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 25.1, 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 25.1,
where f = the frequency of oscillation in MHz, C = the capacitor value in pF, and R = the pull-up resistor
value in k.
Equation 25.1. RC Mode Oscillator Frequency
f = 1.23  10³  R  C
For example: If the frequency desired is 100 kHz, let R = 246 k and C = 50 pF:
f = 1.23( 10³ ) / RC = 1.23 ( 10³ ) / [ 246 x 50 ] = 0.1 MHz = 100 kHz
Referring to the table in SFR Definition 25.4, the required XFCN setting is 010b.
Rev. 0.3
163