User's Manual

Atmel AVR2042

8334A-AVR-05/11

speed. A detailed description of the Atmel ATxmega256A3 can be found in the

datasheet [1].

Table 5-1. ATxmega256A3 ordering information.

Ordering code Flash EEPROM SRAM Speed (MHz) Power supply Package Temperature

ATxmega256A3-AU 256KB + 8KB 4KB 16KB 32 1.6V – 3.6V 64A TQFP-64 -40°C – 85°C

5.3 Clock sources

The XMEGA has a flexible clock system, supporting a large number of clock sources.

It incorporates both calibrated integrated oscillators and external crystal oscillators,

and resonators. The Atmel AVR XMEGA family allows dynamic switching between

the clock sources.

Internal clock sources are:

• 32kHz RC oscillator

• 2MHz RC oscillator

• 32MHz RC oscillator

The 2/32MHz oscillators can be calibrated using an automatic runtime calibration

feature.

In addition to the internal clock sources, two different external clock sources are

supported:

• The 32.768kHz crystal oscillator connected to TOSC1/2 delivers an accurate clock

for a real-time counter, or optionally a system clock for XMEGA

• The transceiver clock, CLKM, can be used as an accurate clock derived from the

16MHz radio transceiver oscillator. This signal is routed to the controller input at

pin 59 (PR1)

A crystal oscillator failure monitor can be enabled to issue a non-maskable interrupt

and switch to internal oscillator if the external oscillator fails.

A high frequency phase-locked loop (PLL) and a clock prescaler are available to

generate a wide range of clock frequencies. 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.

5.3.1 32kHz crystal oscillator

The 32.768kHz crystal oscillator is a low-power oscillator using an external crystal.

The oscillator can be used as a clock source for the system clock, the RTCs, and as a

reference clock for the PLL.

A low-power mode with reduced voltage swing on TOSC2 is available.

The 32kHz crystal is connected to PE6,7.

NOTE These pins cannot be used as general purpose I/O on header PORTE.