ATmega16U4 datasheet(36/408 Pages) ATMEL

36 / 408 page

7766D–AVR–01/09

ATmega16U4/ATmega32U4

• 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

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

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

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

External Clock / Low Power Crystal Oscillator is selected by fuse bits, the SUT fuses are copied

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 Clock / Low Power Crystal 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 Clock / Low Power Crystal Oscillator

The OSCE bit must be written to logic one to enable External Clock / Low Power Crystal Oscilla-

tor. The OSCE bit must be written to logic zero to disable the External Clock / Low Power Crystal

Oscillator.

• Bit 0 – CLKS: Clock Selector

The CLKS bit must be written to logic one to select the External Clock / Low Power Crystal Oscil-

lator 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 Clock / Low Power Crys-

tal Oscillator is selected by the fuse bits configuration.

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

Clock Selection

• 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 Clock / Low Power Crystal Oscillator

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

External Clock / Low Power Crystal 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.

See “Device Clocking Options Select(1)” on page 28 for EXCKSEL[3:0] configuration. Only Low

Power Crystal Oscillator, Calibrated Internal RC Oscillator, and External Clock modes are

allowed.

Clock Status Register

– CLKSTA

• Bit 7-2 - Reserved bits

Bit

RCCKS

EL3

RCCKS

EL2

RCCKS

EL1

RCCKS

EL0

EXCKS

EL3

EXCKS

EL2

EXCKS

EL1

EXCKS

EL0

CLKSEL1

Read/Write

R/W

Initial Value

Bit

RCON

EXTON

CLKSTA

Read/Write

Initial Value

ATmega16U4 Datasheet(PDF) 36 Page - ATMEL Corporation

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