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C8051F060/1/2/3
f = 7.7 / 150 = 0.051 MHz, or 51 kHz
Advanced
Information
14.4.
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 14.1, Option 1. The External Oscillator Frequency Control value (XFCN) should be chosen from
the Crystal column of the table in Figure 14.5 (OSCXCN register). For example, an 11.0592 MHz crystal requires an
XFCN setting of 111b.
When the crystal oscillator is enabled, the oscillator amplitude detection circuit requires a settle time to achieve
proper bias. Introducing a blanking interval of at least 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 proce-
dure is:
Step 1. Enable the external oscillator.
Step 2. Wait at least1 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 and external
noise. 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. Crystal
loading capacitors should be referenced to AGND.
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 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 k
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 14.5, the required XFCN setting is 010.
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 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 )
f = KF / 150
If a frequency of roughly 50 kHz is desired, select the K Factor from the table in Figure 14.5 as KF = 7.7:
Therefore, the XFCN value to use in this example is 010.