81
XMEGA A [MANUAL]
8077I–AVR–11/2012
7.3
Clock Distribution
Figure 7-1 on page 80 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 prescalers that are used to
generate all internal clocks except the asynchronous clock.
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/ClkPER4
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 external 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.
7.4
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 characteristics 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, see “RTCCTRL – RTC Control register” on page 89 for details. 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.
7.4.1.2 32.768kHz Calibrated Oscillator
This oscillator provides an approximate 32.768kHz clock. It is calibrated during production to provide a default frequency
close to its nominal frequency. The calibration register can also be written from software for run-time calibration of the
oscillator frequency. The oscillator employs a built-in prescaler, which provides both a 32.768kHz output and a 1.024kHz
7.4.1.3 32MHz Run-time Calibrated Oscillator
The 32MHz run-time calibrated internal oscillator is a high-frequency oscillator. It is calibrated during production to
provide a default frequency close to its nominal frequency. A digital frequency looked loop (DFLL) can be enabled for
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