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C8051F300 Datasheet(PDF) 97 Page - List of Unclassifed Manufacturers |
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C8051F300 Datasheet(HTML) 97 Page - List of Unclassifed Manufacturers |
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97 / 174 page  C8051F300/1/2/3/4/5 Rev. 2.6 97 11.4. 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 11.1, Option 1. The External Oscillator Frequency Control value (XFCN) should be chosen from the Crystal colum of the table in SFR Definition 11.3 (OSCXCN register). For exam- ple, 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 0’s 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 datasheet 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 11.2. Figure 11.2. 32.768 kHz External Crystal Example 22 pF 22 pF 32.768 kHz 10 M XTAL1 XTAL2 Ω |
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