c8051t617 Silicon Laboratories, c8051t617 Datasheet - Page 115

• Manufacturer Part Number: c8051t617
• Description: Mixed Signal Byte-programmable Eprom Mcu
• Manufacturer: Silicon Laboratories

13.4. 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 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 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
150 kHz:
Since a frequency of roughly 150 kHz is desired, select the K Factor from SFR Definition 13.4 as KF = 22:
Therefore, the XFCN value to use in this example is 011b and C is approximately 50 pF.
13.5. 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 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 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:
Referencing the table in SFR Definition 13.4, the required XFCN setting is 010b.
0.150 MHz
C
0.150 MHz
f
=
f
=
------------------------------------------------ -
0.150 MHz 3.0 V
=
C
-------------------- -
C
1.23 10
-------------------------- -
=
×
KF
R C
48.8 pF
V
=
×
DD
×
=
22
----------------------- -
C 3.0 V
------------------
C
×
×
3
KF
×
22
=
3.0
1.23 10
-------------------------- -
246 50
×
×
3
=
100 kHz
Rev. 0.3
C8051T610/1/2/3/4/5/6/7
DD
= 3.0 V and f =
115