C8051F850-B-GM Datasheet, PDF (115/290 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

C8051F85x/86x
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.
Preliminary Rev 0.6
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