C8051F320-TB Silicon Laboratories Inc, C8051F320-TB Datasheet - Page 120

• Manufacturer Part Number: C8051F320-TB
• Description: BOARD PROTOTYPING W/C8051F320
• Manufacturer: Silicon Laboratories Inc
• Type: MCUr

Specifications of C8051F320-TB

• Contents: Board
• Processor To Be Evaluated: C8051F320/F321
• Interface Type: USB
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: C8051F320
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

C8051F320/1
13.2.3. 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 k and C = 50 pF:
3
3
f = 1.23( 10
) / RC = 1.23 ( 10
) / [ 246 x 50 ] = 0.1 MHz = 100 kHz
Referring to the table in Figure 13.3, the required XFCN setting is 010b. Programming XFCN to a higher
setting in RC mode will improve frequency accuracy at an increased external oscillator supply current.
13.2.4. 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 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 Figure 13.3 as KF = 22:
f = 22 / 150 = 0.146 MHz, or 146 kHz
Therefore, the XFCN value to use in this example is 011b.
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