C8051F314R Silicon Labs, C8051F314R Datasheet - Page 126



Manufacturer Part Number
8-bit Microcontrollers - MCU 8KB 10ADC
Silicon Labs

Specifications of C8051F314R

Product Category
8-bit Microcontrollers - MCU
Data Bus Width
8 bit
Maximum Clock Frequency
25 MHz
Program Memory Size
8 KB
Data Ram Size
1.25 KB
On-chip Adc
Operating Supply Voltage
2.7 V to 3.6 V
Package / Case
Mounting Style
A/d Bit Size
10 bit
A/d Channels Available
Data Rom Size
128 B
Interface Type
Maximum Operating Temperature
+ 85 C
Minimum Operating Temperature
- 40 C
Number Of Programmable I/os
Number Of Timers
On-chip Dac
Processor Series
Program Memory Type
Factory Pack Quantity
Supply Voltage - Max
3.6 V
Supply Voltage - Min
2.7 V
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:
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.
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
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.
Figure 12.2. 32.768 kHz External Crystal Example
32.768 kHz
22 pF
22 pF
Rev. 1.7
10 M

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