C8051F310 datasheet(127/228 Pages) SILABS

127 / 228 page

Rev. 1.7

127

C8051F310/1/2/3/4/5/6/7

12.5. 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 12.1, 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. If the frequency desired is

100 kHz, let R = 246 k

Ω 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 12.4, the required XFCN setting is 010b.

12.6. External Capacitor Example

If a capacitor is used as an external oscillator for the MCU, the circuit should be configured as shown in

Figure 12.1, Option 3. 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 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 from the equations below. Assume VDD = 3.0 V and

C = 50 pF:

f = KF / ( C x VDD ) = KF / ( 50 x 3 ) MHz

f = KF / 150 MHz

If a frequency of roughly 150 kHz is desired, select the K Factor from the table in SFR Definition 12.4 as

KF = 22:

f = 22 / 150 = 0.146 MHz, or 146 kHz

Therefore, the XFCN value to use in this example is 011b.

C8051F310 Datasheet(PDF) 127 Page - Silicon Laboratories