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AT90USB82

8-bit Microcontroller with 8/16K Bytes of ISP Flash and USB Controller

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6.2.4

• 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-

lator is selected by the fuse bits configuration.

The firmware has to check if the clock is correctly started before selected it.

Clock Selection Register 1 – CLKSEL1

Bit

Read/Write

Initial Value

7

RCCKSE

L3

R/W

0

6

RCCKS

EL2

R/W

0

5

RCCLK

SEL1

R/W

1

4

RCCLK

SEL0

R/W

0

3

EXCKSE

L3

R/W

0

2

EXCKSE

L2

R/W

0

1

EXCLKS

EL1

R/W

0

0

EXCLKS

EL0

R/W

0

CLKSEL1

• Bit 7-4 – RCCKSEL[3:0]: CKSEL for RC oscillator

Clock configuration for the RC Oscillator. After a reset, this part of the register is loaded with the

0010b value that corresponds to the RC oscillator. Modifying this value by firmware before

switching to RC oscillator is prohibited because the RC clock will not start.

• Bit 3-0 – EXCKSEL[3:0]: CKSEL for External oscillator / Low Power Oscillator

Clock configuration for the External Oscillator / Low Power Oscillator. After a reset, if the Exter-

nal oscillator / Low Power Oscillator is selected by fuse bits, this part of the register is loaded

with the fuse configuration. Firmware can modify it to change the start-up time after the clock

switch.

28 AT90USB82/162

7707F–AVR–11/10

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