C8051F93X Datasheet, PDF (192/330 Pages) Silicon Laboratories – Pipelined intstruction architecture executes 70 of instruction in 1 or 2 system clocks

C8051F93x-C8051F92x
19.1. Programmable Precision Internal Oscillator
All C8051F93x-C8051F92x 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
“4. Electrical Characteristics” on page 45 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.
19.2. Low Power Internal Oscillator
All C8051F93x-C8051F92x 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
“4. Electrical Characteristics” on page 45 for complete oscillator specifications.
19.3. External Oscillator Drive Circuit
All C8051F93x-C8051F92x 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 19.1
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 “4. Electrical Characteristics” on page 45 for complete oscillator specifications.
19.3.1. 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 19.1, 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 19.2 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 12.5pF 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. Refer to the crystal data
sheet when completing these calculations.
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Rev. 1.3