C8051F330-GP View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

Manufacturer

C8051F330-GP

Mixed Signal ISP Flash MCU Family

Silicon Laboratories 

C8051F330/1/2/3/4/5

13.3.2. 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 13.1, 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 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 13.4, the required XFCN setting is 010b.

13.3.3. External Capacitor Example

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

Figure 13.1, 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 = 150 kHz:

f = KF / (C x VDD)

0.150 MHz = KF / (C x 3.0)

Since the frequency of roughly 150 kHz is desired, select the K Factor from the table in SFR Definition 13.4

(OSCXCN) as KF = 22:

0.150 MHz = 22 / (C x 3.0)

C x 3.0 = 22 / 0.150 MHz

C = 146.6 / 3.0 pF = 48.8 pF

Therefore, the XFCN value to use in this example is 011b and C = 50 pF.

122

Rev. 1.5

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