C8051F970-A-GM Datasheet, PDF (242/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
24.1. Programmable Precision Internal Oscillator
All C8051F97x devices include a programmable precision internal oscillator that may be selected as the system
clock. OSCICL is factory calibrated to obtain a 24.5 MHz frequency. See Section “1. Electrical Characteristics” on
page 10 for complete oscillator specifications.
The precision oscillator supports a spread spectrum mode which modulates the output frequency in order to
reduce the EMI generated by the system. When enabled (SSE = 1), the oscillator output frequency is modulated by
a stepped triangle wave whose frequency is equal to the oscillator frequency divided by 384 (63.8 kHz using the
factory calibration). The deviation from the nominal oscillator frequency is +0%, –1.6%, and the step size is
typically 0.26% of the nominal frequency. When using this mode, the typical average oscillator frequency is
lowered from 24.5 MHz to 24.3 MHz.
24.2. Low Power Internal Oscillator
All C8051F97x devices include a low power internal oscillator that defaults as the system clock after a system
reset. The low power internal oscillator frequency is 20 MHz ± 10% and is automatically enabled when selected as
the system clock and disabled when not in use. See Section “1. Electrical Characteristics” on page 10 for complete
oscillator specifications.
24.3. External Oscillator Drive Circuit
The C8051F970/1/3/4 devices include an external oscillator circuit that may drive an external crystal, ceramic
resonator, capacitor, or RC network. A CMOS clock may also provide a clock input. Figure 24.1 shows a block
diagram of the four external oscillator options. The external oscillator is enabled and configured using the
OSCXCN register.
The external oscillator output may be selected as the system clock or used to clock some of the digital peripherals
(e.g., Timers, PCA, etc.). See the data sheet chapters for each digital peripheral for details. See Section
“1. Electrical Characteristics” on page 10 for complete oscillator specifications.
24.3.1. External Crystal Mode
If a crystal or ceramic resonator is used as the external oscillator, the crystal/resonator and a 10 Mresistor must
be wired across the XTAL1 and XTAL2 pins as shown in Figure 24.1, Option 1. Appropriate loading capacitors
should be added to XTAL1 and XTAL2, and both pins should be configured for analog I/O with the digital output
drivers disabled.
Figure 24.2 shows the external oscillator circuit for a 20 MHz quartz crystal with a manufacturer recommended
load capacitance of 12.5 pF. Loading capacitors are “in-series” as seen by the crystal and “in-parallel” with the stray
capacitance of the XTAL1 and XTAL2 pins. The total value of the each loading capacitor and the stray capacitance
of each XTAL pin should equal 12.5 pF x 2 = 25 pF. With a stray capacitance of 10 pF per pin, the 15 pF capacitors
yield an equivalent series capacitance of 12.5 pF across the crystal.
Note: The recommended load capacitance depends upon the crystal and the manufacturer. Please refer to the crystal data
sheet when completing these calculations.
15 pF
XTAL1
25 MHz
15 pF
10 Mohm
XTAL2
Figure 24.2. 25 MHz External Crystal Example
Rev 1.0
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