Datasheet

21
XMEGA A4U [DATASHEET]
Atmel-8387H-AVR-ATxmega16A4U-34A4U-64A4U-128A4U-Datasheet_09/2014
10. System Clock and Clock options
10.1 Features
z Fast start-up time
z Safe run-time clock switching
z Internal oscillators:
z 32MHz run-time calibrated and tuneable oscillator
z 2MHz run-time calibrated oscillator
z 32.768kHz calibrated oscillator
z 32kHz ultra low power (ULP) oscillator with 1kHz output
z External clock options
z 0.4MHz - 16MHz crystal oscillator
z 32.768kHz crystal oscillator
z External clock
z PLL with 20MHz - 128MHz output frequency
z Internal and external clock options and 1x to 31x multiplication
z Lock detector
z Clock prescalers with 1x to 2048x division
z Fast peripheral clocks running at two and four times the CPU clock
z Automatic run-time calibration of internal oscillators
z External oscillator and PLL lock failure detection with optional non-maskable interrupt
10.2 Overview
Atmel AVR XMEGA A4U 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 10-1 on page 22 presents the principal clock system in the XMEGA A4U 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 24.