C8051F040-TB Silicon Laboratories Inc, C8051F040-TB Datasheet - Page 178

• Manufacturer Part Number: C8051F040-TB
• Description: BOARD PROTOTYPING W/C8051F040
• Manufacturer: Silicon Laboratories Inc
• Type: MCUr

Specifications of C8051F040-TB

• Contents: Board
• Processor To Be Evaluated: C8051F04x
• Interface Type: USB
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: C8051F040
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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:
3
3
f = 1.23( 10
) / RC = 1.23 ( 10
) / [ 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 V
= 3.0 V
DD
and f = 50 kHz:
f = KF / ( C x V
) = KF / ( C x 3 ) = 0.050 MHz
DD
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.
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