Datasheet

21
[Not recommended for new designs - Use XMEGA A1U series] XMEGA A1 [DATASHEET]
8067O–AVR–06/2013
11. System Clock and Clock options
11.1 Features
 Fast start-up time
 Safe run-time clock switching
 Internal Oscillators:
 32 MHz run-time calibrated RC oscillator
 2 MHz run-time calibrated RC oscillator
 32.768 kHz calibrated RC oscillator
 32 kHz Ultra Low Power (ULP) oscillator with 1 kHz ouput
 External clock options
 0.4 - 16 MHz Crystal Oscillator
 32 kHz Crystal Oscillator
 External clock
 PLL with internal and external clock options with 1 to 31x multiplication
 Clock Prescalers with 1x to 2048x division
 Fast peripheral clock running at two and four times the CPU clock speed
 Automatic Run-Time Calibration of internal oscillators
 Crystal Oscillator failure detection
11.2 Overview
Atmel AVR 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. 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 11-1 on page 22 presents the principal clock system in the XMEGA A1U family 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.