C8051F340/1/2/3/4/5/6/7
14.3.3. 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 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
Ω
and C = 50 pF:
3
3
1.23
(
10
)
1.23
(
10
)
-
-
f
= ----------------------- = ------------------------- = 0.1 MHz = 100 kHz
RC
[
246
×
50
]
Referring to the table in SFR Definition 14.4, 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 cur-
rent.
14.3.4. External Capacitor Example
If a capacitor is used as an external oscillator for the MCU, the circuit should be configured as shown in
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 V
DD
= 3.0 V and C =
50 pF:
KF
KF
-
f
= ------------------------- = -------------------------------
(
C
×
V
DD
)
(
50 x 3
)MHz
KF
f
= ----------------------
150 MHz
If a frequency of roughly 150 kHz is desired, select the K Factor from the table in SFR Definition 14.4 as
KF = 22:
22-
f
= -------- = 0.146 MHz, or 146 kHz
150
Therefore, the XFCN value to use in this example is 011b.
144
Rev. 1.0
SILABS [ SILICON LABORATORIES ]
