C8051F336_08 Datasheet, PDF (117/227 Pages) Silicon Laboratories – Mixed-Signal Byte-Programmable EPROM MCU

C8051F336/7/8/9
10MΩ
XTAL1
XTAL2
22pF*
32.768 kHz
22pF*
* Capacitor values depend on
crystal specifications
Figure 19.2. 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 19.1,
where f = the frequency of oscillation in MHz, C = the capacitor value in pF, and R = the pull-up resistor
value in kΩ.
Equation 19.1. RC Mode Oscillator Frequency
f = 1.23 × 10³ ⁄ (R × C)
For example: If the frequency desired is 100 kHz, let R = 246 kΩ 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 19.5, the required XFCN setting is 010b.
Rev.1.0
117