C8051F330-GMR Datasheet, PDF (114/210 Pages) Silicon Laboratories – Mixed-Signal ISP Flash MCU

C8051F330/1/2/3/4/5
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 SFR Definition 13.4 (OSCXCN register). For
example, an 11.0592 MHz crystal requires an XFCN setting of 111b and a 32.768 kHz Watch Crystal
requires an XFCN setting of 001b. After an external 32.768 kHz oscillator is stabilized, the XFCN setting
can be switched to 000 to save power. It is recommended to enable the missing clock detector before
switching the system clock to any external oscillator source.
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 rec-
ommended procedure is:
Step 1. Force XTAL1 and XTAL2 to a low state. This involves enabling the Crossbar and writing ‘0’
to port latches P0.2 and P0.3.
Step 2. Configure XTAL1 and XTAL2 as analog inputs using register P0MDIN.
Step 3. Enable the external oscillator.
Step 4. Wait at least 1 ms.
Step 5. Poll for XTLVLD => ‘1’.
Step 6. Enable the Missing Clock Detector.
Step 7. 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.
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