C8051F850-C-GM Datasheet, PDF (136/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

16.3. External Clock
An external CMOS clock source is also supported by the C8051F85x/86x family. The EXTCLK pin on the
device serves as the external clock input when running in this mode. The EXTCLK input may also be used
to clock some of the digital peripherals (e.g., Timers, PCA, etc.) while SYSCLK runs from one of the
internal oscillator sources. When not selected as the SYSCLK source, the EXTCLK input is always re-
synchronized to SYSCLK.
16.4. Clock Selection
The CLKSEL register is used to select the clock source for the system. The CLKSL field selects which
oscillator source is used as the system clock, while CLKDIV controls the programmable divider. CLKSL
must be set to 01b 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. In these cases, the external oscillator source is synchronized to the SYSCLK source. The system
clock may be switched on-the-fly between any of the oscillator sources so long as the selected clock
source is enabled and has settled, and CLKDIV may be changed at any time.
The internal high-frequency and low-frequency oscillators require little start-up time and may be selected
as the system clock immediately following the register write which enables the oscillator. When selecting
the EXTCLK pin as a clock input source, the pin should be skipped in the crossbar and configured as a
digital input. Firmware should ensure that the external clock source is present or enable the missing clock
detector before switching the CLKSL field.
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