C8051F360 datasheet(177/288 Pages) SILABS

177 / 288 page

Rev. 1.0

177

C8051F360/1/2/3/4/5/6/7/8/9

16.6. 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 16.1, 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 16.5, the required XFCN setting is 010b. Programming XFCN to a

higher setting in RC mode will improve frequency accuracy at a slightly increased external oscillator supply

current.

16.7. External Capacitor Example

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

Figure 16.1, 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

f = 75 kHz:

f = KF / (C x VDD)

0.075 MHz = KF / (C x 3.0)

Since the frequency of roughly 75 kHz is desired, select the K Factor from the table in SFR Definition 16.5

as KF = 7.7:

0.075 MHz = 7.7 / (C x 3.0)

C x 3.0 = 7.7 / 0.075 MHz

C = 102.6 / 3.0 pF = 34.2 pF

Therefore, the XFCN value to use in this example is 010b.

C8051F360 Datasheet(PDF) 177 Page - Silicon Laboratories