Datasheet

37
7799D–AVR–11/10
ATmega8U2/16U2/32U2
Figure 8-6. PLL Clocking System
8.11 Register Description
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
Tclk
Timer1
To System
Clock Prescaler
clk
8MHz
PLL clock
Prescaler
PINDIV
PDIV3..0
clk
USB
/2
/48
PLLITM
PLLUSB
0
1
0
1
CKSEL3:0
PLOCK
T1
Bit 7 6 5 4 3 2 1 0
(0xD0) RCSUT1 RCSUT0 EXSUT1 EXSUT0 RCE EXTE - CLKS CLKSEL0
Read/Write R/W R/W R/W R/W R/W R/W R R/W
Initial Value 0 0 0 0 See Bit Description