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8331B–AVR–03/12
Atmel AVR XMEGA AU
7.3
Clock Distribution
presents the principal clock distribution system used in XMEGA devices.
7.3.1
System Clock - Clk
SYS
The system clock is the output from the main system clock selection. This is fed into the prescal-
ers that are used to generate all internal clocks except the asynchronous and USB clocks.
7.3.2
CPU Clock - Clk
CPU
The CPU clock is routed to the CPU and nonvolatile memory. Halting the CPU clock inhibits the
CPU from executing instructions.
7.3.3
Peripheral Clock - Clk
PER
The majority of peripherals and system modules use the peripheral clock. This includes the DMA
controller, event system, interrupt controller, external bus interface and RAM. This clock is
always synchronous to the CPU clock, but may run even when the CPU clock is turned off.
7.3.4
Peripheral 2x/4x Clocks - Clk
PER2
/Clk
PER4
Modules that can run at two or four times the CPU clock frequency can use the peripheral 2x
and peripheral 4x clocks.
7.3.5
Asynchronous Clock - Clk
RTC
The asynchronous clock allows the real-time counter (RTC) to be clocked directly from an exter-
nal 32.768kHz crystal oscillator or the 32 times prescaled output from the internal 32.768kHz
oscillator or ULP oscillator. The dedicated clock domain allows operation of this peripheral even
when the device is in sleep mode and the rest of the clocks are stopped.
7.3.6
USB Clock - Clk
USB
The USB device module requires a 12MHz or 48MHz clock. It has a separate clock source
selection in order to avoid system clock source limitations when USB is used.
7.4
Clock Sources
The clock sources are divided in two main groups: internal oscillators and external clock
sources. Most of the clock sources can be directly enabled and disabled from software, while
others are automatically enabled or disabled, depending on peripheral settings. After reset, the
device starts up running from the 2MHz internal oscillator. The other clock sources, DFLLs and
PLL, are turned off by default.
7.4.1
Internal Oscillators
The internal oscillators do not require any external components to run. For details on character-
istics and accuracy of the internal oscillators, refer to the device datasheet.
7.4.1.1
32kHz Ultra Low Power Oscillator
This oscillator provides an approximate 32kHz clock. The 32kHz ultra low power (ULP) internal
oscillator is a very low power clock source, and it is not designed for high accuracy.The oscillator
employs a built-in prescaler that provides a 1kHz output. The oscillator is automatically
enabled/disabled when it is used as clock source for any part of the device. This oscillator can
be selected as the clock source for the RTC.
