ATXMEGA128B1 Datasheet, PDF (19/138 Pages) ATMEL Corporation – 8/16-bit Atmel XMEGA B1 Microcontroller

Figure 10-1. The Clock system, clock sources and clock distribution.
LCD
Real Time
Counter
Peripherals
RAM
AVR CPU
Non-Volatile
Memory
clkLCD
clkRTC
Watchdog
Timer
Brown-out
Detector
RTCSRC
clkPER
clkPER2
clkPER4
System Clock Prescalers
clkSYS
System Clock Multiplexer
(SCLKSEL)
clkCPU
USB
clkUSB
Prescaler
USBSRC
PLL
PLLSRC
XOSCSEL
32 kHz
Int. ULP
32.768 kHz
Int. OSC
32.768 kHz
TOSC
0.4 – 16 MHz
XTAL
32 MHz
Int. Osc
2 MHz
Int. Osc
10.3
Clock Sources
The clock sources are divided in two main groups: internal oscillators and external clock sources. Most of the clock
sources can be directly enabled and disabled from software, while others are automatically enabled or disabled,
depending on peripheral settings. After reset, the device starts up running from the 2MHz internal oscillator. The other
clock sources, DFLLs and PLL, are turned off by default.
The internal oscillators do not require any external components to run. For details on characteristics and accuracy of the
internal oscillators, refer to the device datasheet.
10.3.1 32kHz Ultra Low Power Internal Oscillator
This oscillator provides an approximate 32kHz clock. The 32kHz ultra low power (ULP) internal oscillator is a very low
power clock source, and it is not designed for high accuracy.The oscillator employs a built-in prescaler that provides a
XMEGA B1 [DATASHEET] 19
8330C–AVR–07/2012