C8051F310 datasheet(126/228 Pages) SILABS

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C8051F310/1/2/3/4/5/6/7

126

Rev. 1.7

12.4. 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 12.1, Option 1. The External Oscillator Frequency Control value (XFCN)

should be chosen from the Crystal column of the table in SFR Definition 12.4. 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 rec-

ommended procedure is:

Step 1. Force the XTAL1 and XTAL2 pins low by writing 0s to the port latch.

Step 2. Configure XTAL1 and XTAL2 as analog inputs.

Step 3. Enable the external oscillator.

Step 4. Wait at least 1 ms.

Step 5. Poll for XTLVLD => '1'.

Step 6. Switch the system clock to the external oscillator.

Note: Tuning-fork crystals may require additional settling time before XTLVLD returns a valid result.

The capacitors shown in the external crystal configuration provide the load capacitance required by the

crystal for correct oscillation. These capacitors are "in series" as seen by the crystal and "in parallel" with

the stray capacitance of the XTAL1 and XTAL2 pins.

Note: The load capacitance depends upon the crystal and the manufacturer. Please refer to the crystal

data sheet when completing these calculations.

For example, a tuning-fork crystal of 32.768 kHz with a recommended load capacitance of 12.5 pF should

use the configuration shown in Figure 12.1, Option 1. The total value of the capacitors and the stray capac-

itance of the XTAL pins should equal 25 pF. With a stray capacitance of 3 pF per pin, the 22 pF capacitors

yield an equivalent capacitance of 12.5 pF across the crystal, as shown in Figure 12.2.

Figure 12.2. 32.768 kHz External Crystal Example

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.

22 pF

32.768 kHz

10 M

XTAL1

XTAL2

Ω

C8051F310 Datasheet(PDF) 126 Page - Silicon Laboratories