Specifications

C8051F330/1
Rev. 1.1 111
13.3.1. External Crystal Example
If a crystal or ceramic resonator is used as an external oscillator source for the MCU, the circuit should be configured
as shown in
Figure 13.1, Option 1. The External Oscillator Frequency Control value (XFCN) should be chosen from
the Crystal column of the table in Figure 13.5 (OSCXCN register). For example, an 11.0592 MHz crystal requires an
XFCN setting of 111b.
When the crystal oscillator is first enabled, the oscillator amplitude detection circuit requires a settling time to
achieve proper bias. Introducing a delay of 1
ms between enabling the oscillator and checking the XTLVLD bit will
prevent a premature switch to the external oscillator as the system clock. Switching to the external oscillator before
the crystal oscillator has stabilized can result in unpredictable behavior. The recommended procedure is:
Step 1. Enable the external oscillator.
Step 2. Wait at least 1 ms.
Step 3. Poll for XTLVLD => ‘1’.
Step 4. Switch the system clock to the external oscillator.
Important Note on External Crystals: Crystal oscillator circuits are quite sensitive to PCB layout. The crystal
should be placed as close as possible to the XTAL pins on the device. The traces should be as short as possible and
shielded with ground plane from any other traces which could introduce noise or interference.
13.3.2. 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 determine the required External Oscil-
lator 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 kΩ and C = 50 pF:
f = 1.23( 10
3
) / RC = 1.23 ( 10
3
) / [ 246 * 50 ] = 0.1 MHz = 100 kHz
Referring to the table in Figure 13.5, the required XFCN setting is 010b.
13.3.3. External Capacitor Example
If a capacitor is used as an external oscillator for the MCU, the circuit should be configured as shown in Figure 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 Fre
-
quency Control value (XFCN) in the OSCXCN Register, select the capacitor 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 * VDD ) = KF / ( 50 * 3 ) MHz
f = KF / 150 MHz
If a frequency of roughly 150 kHz is desired, select the K Factor from the table in Figure 13.5 as KF = 22:
f = 22 / 150 = 0.146 MHz, or 146 kHz
Therefore, the XFCN value to use in this example is 011b.
13.4. System Clock Selection
The CLKSL bits in register OSCICN select which oscillator is used as the system clock. CLKSL0 must be set to ‘1’
for the system clock to run from the external oscillator; however the external oscillator may still clock certain periph
-