• 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.
6.2.4
Clock Selection Register 1 – CLKSEL1
Bit
7
6
5
4
3
2
1
0
RCCKSE
L3
RCCKS
EL2
RCCLK
SEL1
RCCLK
SEL0
EXCKSE
L3
EXCKSE
L2
EXCLKS
EL1
EXCLKS
EL0
CLKSEL1
Read/Write
Initial Value
R/W
0
R/W
0
R/W
1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
• 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
7707D–AVR–07/08
ATMEL [ ATMEL ]
