ATXMEGA256A3U Datasheet, PDF (18/116 Pages) ATMEL Corporation – 8/16-bit Atmel XMEGA A3U Microcontroller

XMEGA A3U
10. System Clock and Clock options
10.1 Features
10.2 Overview
• Fast start-up time
• Safe run-time clock switching
• Internal Oscillators:
– 32MHz run-time calibrated and tuneable oscillator
– 2MHz run-time calibrated oscillator
– 32.768kHz calibrated oscillator
– 32kHz Ultra Low Power (ULP) oscillator with 1kHz output
• External clock options
– 0.4 - 16MHz Crystal Oscillator
– 32kHz Crystal Oscillator
– External clock
• PLL with 20 - 128MHz output frequency
– Internal and external clock options and 1 to 31x multiplication
– Lock detector
• Clock Prescalers with 1 to 2048x division
• Fast peripheral clocks running at 2 and 4 times the CPU clock frequency
• Automatic Run-Time Calibration of internal oscillators
• External oscillator and PLL lock failure detection with optional non maskable interrupt
The flexible clock system supports a large number of clock sources. It incorporates both accu-
rate internal oscillators, and external crystal oscillators 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 runtime
calibration of the internal oscillators to remove frequency drift over voltage and temperate. An
Oscillator Failure Monitor can be enabled to issue a Non-Maskable Interrupt and switch to inter-
nal oscillator if the external oscillator or PLL fails.
When a reset occur, all clock sources except 32kHz Ultra Low Power oscillator are disabled.
After reset, the device will always start up running from the 2MHz internal oscillator. During nor-
mal operation, the System Clock source and prescalers can be changed from software at any
time.
Figure 10-1 on page 19 presents the principal clock system in the XMEGA. All of the clocks do
not need to be active at a given time. The clocks to the CPU and peripherals can be stopped
using sleep modes and power reduction registers.
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8386A–AVR–07/11