C8051F045 View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

Manufacturer

C8051F045

Mixed Signal ISP Flash MCU Family

Silicon Laboratories 

C8051F040/1/2/3/4/5/6/7

14.5. 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 14.1, 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 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 14.4, the required XFCN setting is 010b.

14.6. External Capacitor Example

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

Figure 14.1, 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 desired

frequency of oscillation and find the capacitor to be used from the equations below. Assume VDD = 3.0 V

and f = 50 kHz:

f = KF / ( C x VDD ) = KF / ( C x 3 ) = 0.050 MHz

If a frequency of roughly 50 kHz is desired, select the K Factor from the table in SFR Definition 14.4 as

KF = 7.7:

0.050 MHz = 7.7 / (C x 3)

C x 3 = 7.7 / 0.050 = 154, so C = 154 / 3 pF = 51.3 pF

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

178

Rev. 1.5

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