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8068C–AVR–06/08
XMEGA A3
10. System Clock and Clock options
10.1 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 kHz calibrated RC oscillator
– 32 kHz Ultra Low Power (ULP) oscillator
• External clock options
– 0.4 - 16 MHz Crystal Oscillator
– 32 kHz Crystal Oscillator
– External clock
• PLL with internal and external clock options with 2 to 31x multiplication
• Clock Prescalers with 2 to 2048x division
• Fast peripheral clock running at 2 and 4 times the CPU clock speed
• Automatic Run-Time Calibration of internal oscillators
• Crystal Oscillator failure detection
10.2 Overview
XMEGA A3 has an advanced clock system, supporting a large number of clock sources. It incor-
porates both integrated oscillators, external crystal oscillators and resonators. A high frequency
Phase Locked Loop (PLL) and clock prescalers can be controlled from software to generate a
wide range of clock frequencies from the clock source input.
It is possible to switch between clock sources from software during run-time. After reset the
device will always start up running from the 2 Mhz internal oscillator.
A calibration feature is available, and can be used for automatic run-time calibration of the inter-
nal 2 MHz and 32 MHz oscillators. This reduce frequency drift over voltage and temperature.
A Crystal Oscillator Failure Monitor can be enabled to issue a Non-Maskable Interrupt and
switch to internal oscillator if the external oscillator fails. Figure 10-1 on page 18 shows the prin-
cipal clock system in XMEGA A3.
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