18
8068C–AVR–06/08
XMEGA A3
Figure 10-1. Clock system overview
Each clock source is briefly described in the following sub-sections.
10.3 Clock Options
10.3.1 32 kHz Ultra Low Power Internal Oscillator
The 32 kHz Ultra Low Power (ULP) Internal Oscillator is a very low power consumption clock
source. It is used for the Watchdog Timer, Brown-Out Detection and as an asynchronous clock
source for the Real Time Counter. This oscillator cannot be used as the system clock source,
and it cannot be directly controlled from software.
10.3.2 32.768 kHz Calibrated Internal Oscillator
The 32.768 kHz Calibrated Internal Oscillator is a high accuracy clock source that can be used
as the system clock source or as an asynchronous clock source for the Real Time Counter. It is
calibrated during protection to provide a default frequency which is close to its nominal
frequency.
32 MHz
Run-time Calibrated
Internal Oscillator
32 kHz ULP
Internal Oscillator
32.768 kHz
Calibrated Internal
Oscillator
32.768 KHz
Crystal Oscillator
0.4 - 16 MHz
Crystal Oscillator
2 MHz
Run-Time Calibrated
Internal Oscillator
External
Clock Input
CLOCK CONTROL
UNIT
with PLL and
Prescaler
WDT/BOD
clk
ULP
RTC
clk
RTC
EVSYS
PERIPHERALS
ADC
DAC
PORTS
...
clk
PER
DMA
INTERRUPT
RAM
NVM MEMORY
FLASH
EEPROM
CPU
clk
CPU
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