8. Clocking and Oscillators
8.1 Introduction
The CPU core and peripheral subsystem may be clocked by both internal and external oscillator resources. By default, the system
clock comes up running from the 20 MHz low power oscillator divided by 8.
Clock Control
Programmable
Divider:
1, 2, 4...128
SYSCLK
24.5 MHz
Oscillator
(HFOSC0)
To core and peripherals
Low Power
Oscillator
(LPOSC0)
/8
RTC Oscillator
(RTCOSC)
External Oscillator
Input (EXTCLK)
Figure 8.1. Clock Control Block Diagram
8.2 Features
• Provides clock to core and peripherals.
• 20 MHz low power oscillator (LPOSC0), accurate to +/- 10% over supply and temperature corners.
• 24.5 MHz internal oscillator (HFOSC0), accurate to +/- 2% over supply and temperature corners.
• External RTC 32 kHz crystal.
• External RC, C, CMOS, and high-frequency crystal clock options (EXTCLK).
• Clock divider with eight settings for flexible clock scaling: Divide the selected clock source by 1, 2, 4, 8, 16, 32, 64, or 128.
8.3 Functional Description
8.3.1 Clock Selection
The CLKSEL register is used to select the clock source for the system (SYSCLK). The CLKSL field selects which oscillator source is
used as the system clock, while CLKDIV controls the programmable divider. When an internal oscillator source is selected as the
SYSCLK, the external oscillator may still clock certain peripherals. 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.
Note:
Some device families do place restrictions on the difference in operating frequency when switching clock sources. Please see the
CLKSEL register description for details.
8.3.2 LPOSC0 20 MHz Internal Oscillator
LPOSC0 is a programmable internal low power oscillator that is factory-calibrated to 20 MHz. The oscillator is automatically enabled
when selected as the system clock and disabled when not in use. This oscillator tolerance is ±10%.
8.3.3 HFOSC0 24.5 MHz Internal Oscillator
HFOSC0 is a programmable internal high-frequency oscillator that is factory-calibrated to 24.5 MHz. The oscillator is automatically ena-
bled when it is requested. The oscillator period can be adjusted via the HFO0CAL register to obtain other frequencies.
EFM8SB2 Reference Manual
Clocking and Oscillators
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