C8051F310 datasheet(124/228 Pages) SILABS

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

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Rev. 1.7

12.2. External Oscillator Drive Circuit

The external oscillator circuit may drive an external crystal, ceramic resonator, capacitor, or RC network. A

CMOS clock may also provide a clock input. For a crystal or ceramic resonator configuration, the crys-

tal/resonator must be wired across the XTAL1 and XTAL2 pins as shown in Option 1 of Figure 12.1. A

10 M

Ω resistor also must be wired across the XTAL2 and XTAL1 pins for the crystal/resonator configura-

tion. In RC, capacitor, or CMOS clock configuration, the clock source should be wired to the XTAL2 pin as

shown in Option 2, 3, or 4 of Figure 12.1. The type of external oscillator must be selected in the OSCXCN

Important Note on External Oscillator Usage: Port pins must be configured when using the external

oscillator circuit. When the external oscillator drive circuit is enabled in crystal/resonator mode, Port pins

P0.2 and P0.3 are used as XTAL1 and XTAL2 respectively. When the external oscillator drive circuit is

enabled in capacitor, RC, or CMOS clock mode, Port pin P0.3 is used as XTAL2. The Port I/O Crossbar

should be configured to skip the Port pins used by the oscillator circuit; see Section “13.1. Priority Cross-

bar Decoder” on page 131 for Crossbar configuration. Additionally, when using the external oscillator cir-

cuit in crystal/resonator, capacitor, or RC mode, the associated Port pins should be configured as analog

inputs. In CMOS clock mode, the associated pin should be configured as a digital input. See Section

“13.2. Port I/O Initialization” on page 133 for details on Port input mode selection.

12.3. System Clock Selection

The CLKSL0 bit in register CLKSEL selects which oscillator is used as the system clock. CLKSL0 must be

set to ‘1’ for the system clock to run from the external oscillator; however the external oscillator may still

clock certain peripherals (timers, PCA) when the internal oscillator is selected as the system clock. The

system clock may be switched on-the-fly between the internal and external oscillator, so long as the

selected oscillator is enabled and has settled. The internal oscillator requires little start-up time and may be

selected as the system clock immediately following the OSCICN write that enables the internal oscillator.

External crystals and ceramic resonators typically require a start-up time before they are settled and ready

for use as the system clock. The Crystal Valid Flag (XTLVLD in register OSCXCN) is set to ‘1’ by hardware

when the external oscillator is settled. To avoid reading a false XTLVLD, in crystal mode software

should delay at least 1 ms between enabling the external oscillator and checking XTLVLD. RC and

C modes typically require no startup time.

C8051F310 Datasheet(PDF) 124 Page - Silicon Laboratories