C8051F38B-GMR 查看數據表（PDF） - Silicon Laboratories

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C8051F38B-GMR

Full Speed USB Flash MCU Family

Silicon Laboratories 

C8051F380/1/2/3/4/5/6/7/C

19.6.2. 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, “RC Mode”. 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 , where

f = the frequency of oscillation in MHz, C = the capacitor value in pF, and R = the pull-up resistor value in

k.

f = 1.23  103 § R  C

Equation 19.1. RC Mode Oscillator Frequency

For example: 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 19.6, the required XFCN setting is 010b.

19.6.3. External Capacitor Example

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

Figure 19.1, “C Mode”. The capacitor should be no greater than 100 pF; however, for very small capaci-

tors, 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 according to Equation , where f = the frequency of oscil-

lation in MHz, C = the capacitor value in pF, and VDD = the MCU power supply in Volts.

f = KF  C  VDD

Equation 19.2. C Mode Oscillator Frequency

For example: 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 19.6

(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.

Rev. 1.4

151

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