C8051F99X_10 Datasheet, PDF (186/322 Pages) Silicon Laboratories – Ultra Low Power, 8-2 kB Flash, Capacitive Sensing MCU

C8051F99x-C8051F98x
19. Clocking Sources
C8051F99x-C8051F98x devices include a programmable precision internal oscillator, an external oscillator
drive circuit, a low power internal oscillator, and a SmaRTClock real time clock oscillator. The precision
internal oscillator can be enabled/disabled and calibrated using the OSCICN and OSCICL registers, as
shown in Figure 19.1. The external oscillator can be configured using the OSCXCN register. The low
power internal oscillator is automatically enabled and disabled when selected and deselected as a clock
source. SmaRTClock operation is described in the SmaRTClock oscillator chapter.
The system clock (SYSCLK) can be derived from the precision internal oscillator, external oscillator, low
power internal oscillator, low power internal oscillator divided by 8, or SmaRTClock oscillator. The global
clock divider can generate a system clock that is 1, 2, 4, 8, 16, 32, 64, or 128 times slower that the selected
input clock source. Oscillator electrical specifications can be found in the Electrical Specifications Chapter.
Option 2
VDD
Option 3
XTAL2
XTAL2
Option 1
XTAL1
10M
XTAL2
Option 4
XTAL2
OSCICL
OSCICN
CLKSEL
EN
Precision
Internal Oscillator
External
Oscillator
Drive Circuit
Precision Internal Oscillator
External Oscillator
Low Power Internal Oscillator
Low Power Internal
Oscillator Divided by 8
8
SmaRTClock Oscillator
CLKRDY
n
Clock Divider
SYSCLK
Low Power
Internal Oscillator
SmaRTClock
Oscillator
OSCXCN
Figure 19.1. Clocking Sources Block Diagram
The proper way of changing the system clock when both the clock source and the clock divide value are
being changed is as follows:
If switching from a fast “undivided” clock to a slower “undivided” clock:
1. Change the clock divide value.
2. Poll for CLKRDY > 1.
3. Change the clock source.
If switching from a slow “undivided” clock to a faster “undivided” clock:
1. Change the clock source.
2. Change the clock divide value.
3. Poll for CLKRDY > 1.
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