C8051F700DK Silicon Laboratories Inc, C8051F700DK Datasheet - Page 179

• Manufacturer Part Number: C8051F700DK
• Description: DEV KIT FOR C8051F700
• Manufacturer: Silicon Laboratories Inc
• Type: MCUr

Specifications of C8051F700DK

• Contents: Board, Cables, CD, Debugger, Power Supply
• Processor To Be Evaluated: C8051F7x
• Processor Series: C8051F7xx
• Interface Type: USB
• Operating Supply Voltage: 3.3 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: C8051F7xx
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 336-1635

C8051F70x/71x
27.3.3. External Capacitor Example
If a capacitor is used as an external oscillator for the MCU, the circuit should be configured as shown in
Figure 27.1, 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 according to Equation , where f = the frequency of oscil-
lation in MHz, C = the capacitor value in pF, and V
= the MCU power supply in Volts.
DD






f
=
KF
R V
DD
Equation 27.2. C Mode Oscillator Frequency
For example: Assume V
= 3.0 V and f = 150 kHz:
DD
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 27.4
(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.
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