ATmega32U2 datasheet(37/314 Pages) ATMEL

37 / 314 page

7799B–AVR–07/09

ATmega8U2/16U2/32U2

Figure 8-6.

PLL Clocking System

8.11

8.11.1

CLKSEL0 – Clock Selection Register 0

• Bit 7:6 – RCSUT[1:0]: SUT for RC oscillator

These 2 bits are the SUT value for the RC Oscillator. If the RC oscillator is selected by fuse bits,

the SUT fuse are copied into these bits. A firmware change will not have any effect because this

additionnal start-up time is only used after a reset and not after a clock switch.

• Bit 5:4 – EXSUT[1:0]: SUT for External Oscillator / Low Power Oscillator

These 2 bits are the SUT value for the External Oscillator / Low Power Oscillator. If the External

oscillator / Low Power Oscillator is selected by fuse bits, the SUT fuse are copyed into these

bits. The firmware can modify these bits by writing a new value. This value will be used at the

next start of the External Oscillator / Low Power Oscillator.

• Bit 3 – RCE: Enable RC Oscillator

The RCE bit must be written to logic one to enable the RC Oscillator. The RCE bit must be writ-

ten to logic zero to disable the RC Oscillator.

• Bit 2 – EXTE: Enable External Oscillator / Low Power Oscillator

The OSCE bit must be written to logic one to enable External Oscillator / Low Power Oscillator.

The OSCE bit must be written to logic zero to disable the External Oscillator / Low Power

Oscillator.

• Bit 0 – CLKS: Clock Selector

The CLKS bit must be written to logic one to select the External Oscillator / Low Power Oscillator

as CPU clock. The CLKS bit must be written to logic zero to select the RC Oscillator as CPU

clock. After a reset, the CLKS bit is set by hardware if the External Oscillator / Low Power Oscil-

8 MHz

RC OSCILLATOR

XTAL1

XTAL2

XTAL

OSCILLATOR

PLL

PLLE

Lock

Detector

TclkTimer1

To System

Clock Prescaler

clk

8MHz

PLL clock

Prescaler

PINDIV

PDIV3..0

clkUSB

/48

PLLITM

PLLUSB

CKSEL3:0

PLOCK

Bit

654

(0xD0)

RCSUT1

RCSUT0

EXSUT1

EXSUT0

RCE

EXTE

CLKS

CLKSEL0

Read/Write

R/W

Initial Value

See Bit Description

ATmega32U2 Datasheet(PDF) 37 Page - ATMEL Corporation

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