Atmel XMEGA B, User Manual

Page: 77 / 415

77 XMEGA B [DATASHEET]
8291B–AVR–01/2013
7. System Clock and Clock Options
7.1 Features

Fast start-up time

Safe run-time clock switching

Internal oscillators:

32MHz run-time calibrated oscillator

2MHz run-time calibrated oscillator

32.768kHz calibrated oscillator

32kHz ultra low power (ULP) oscillator with 1kHz output

External clock options

0.4MHz - 16MHz crystal oscillator

32.768kHz crystal oscillator

External clock

PLL with 20MHz - 128MHz output frequency

Internal and external clock options and 1x to 31x multiplication

Lock detector
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Clock prescalers with 1x to 2048x division

Fast peripheral clocks running at 2 and 4 times the CPU clock

Automatic run-time calibration of internal oscillators

External oscillator and PLL lock failure detection with optional non-maskable interrupt
7.2 Overview
XMEGA devices have a flexible clock system supporting a large number of clock sources. It incorporates both accurate
internal oscillators and external crystal oscillator and resonator support. A high-frequency phase locked loop (PLL) and
clock prescalers can be used to generate a wide range of clock frequencies. A calibration feature (DFLL) is available,
and can be used for automatic run-time calibration of the internal oscillators to remove frequency drift over voltage and
temperature. An oscillator failure monitor can be enabled to issue a non-maskable interrupt and switch to the internal
oscillator if the external oscillator or PLL fails.
When a reset occurs, all clock sources except the 32kHz ultra low power oscillator are disabled. After reset, the device
will always start up running from the 2MHz internal oscillator. During normal operation, the system clock source and
prescalers can be changed from software at any time.
Figure 7-1 on page 78 presents the principal clock system in the XMEGA family of devices. Not all of the clocks need to
be active at a given time. The clocks for the CPU and peripherals can be stopped using sleep modes and power
reduction registers, as described in
“Power Management and Sleep Modes” on page 97 .