C8051F02X Datasheet, PDF (138/272 Pages) Silicon Laboratories – 8K ISP FLASH MCU Family

C8051F020/1/2/3
14.1. External Crystal Example
If a crystal or ceramic resonator is used as an external oscillator source for the MCU, the circuit should be as shown
in Figure 14.1, Option 1. The External Oscillator Frequency Control value (XFCN) should be chosen from the Crys-
tal column of the table in Figure 14.3 (OSCXCN register). For example, an 11.0592 MHz crystal requires an XFCN
setting of 111b.
The Crystal Oscillator Valid Flag (XTLVLD in register OSCXCN) is set to logic 1 by hardware when the external
crystal oscillator is running and stable. The XTLVLD detection circuit requires a startup time of at least 1 ms between
enabling the oscillator and checking the XTLVLD bit. 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: 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, as should the loading capacitors on the crystal pins. The traces should be as
short as possible and shielded with ground plane from any other traces which could introduce noise or interference.
14.2. External RC Example
If an RC network is used as an external oscillator source for the MCU, the circuit should be as shown in Figure 14.1,
Option 2. The capacitor must be no greater than 100 pF; however for small capacitors (less than ~20 pF), the total
capacitance may be dominated by PWB parasitic capacitance. To determine the required External Oscillator Fre-
quency Control value (XFCN) in the OSCXCN Register, first select the RC network value to produce the desired fre-
quency of oscillation. If the frequency desired is 100 kHz, let R = 246 kΩ and C = 50 pF:
f = 1.23( 10³ ) / RC = 1.23 ( 10³ ) / [ 246 * 50 ] = 0.1 MHz = 100 kHz
XFCN ≥ log₂ ( f / 25 kHz )
XFCN ≥ log₂ ( 100 kHz / 25 kHz ) = log₂ ( 4 )
XFCN ≥ 2, or code 010b
14.3. External Capacitor Example
If a capacitor is used as an external oscillator for the MCU, the circuit should be as shown in Figure 14.1, Option 3.
The capacitor must be no greater than 100 pF; however for small capacitors (less than ~20 pF), the total capacitance
may be dominated by PWB parasitic capacitance. To determine the required External Oscillator Frequency 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 90 kHz is desired, select the K Factor from the table in Figure 14.3 as KF = 13:
f = 13 / 150 = 0.087 MHz, or 87 kHz
Therefore, the XFCN value to use in this example is 011b.
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Rev. 1.4