92
8331B–AVR–03/12
Atmel AVR XMEGA AU
7.9
Register Description – Clock
7.9.1
CTRL – Control register
• Bit 7:3 – Reserved
These bits are unused and reserved for future use. For compatibility with future devices, always
write these bits to zero when this register is written.
• Bit 2:0 – SCLKSEL[2:0]: System Clock Selection
These bits are used to select the source for the system clock. See
for the different
selections. Changing the system clock source will take two clock cycles on the old clock source
and two more clock cycles on the new clock source. These bits are protected by the configura-
tion change protection mechanism. For details, refer to
”Configuration Change Protection” on
SCLKSEL cannot be changed if the new clock source is not stable. The old clock can not be dis-
abled until the clock switching is completed.
7.9.2
PSCTRL – Prescaler register
This register is protected by the configuration change protection mechanism. For details, refer to
”Configuration Change Protection” on page 13
.
• Bit 7 – Reserved
This bit is unused and reserved for future use. For compatibility with future devices, always write
this bit to zero when this register is written.
Bit
7
6
5
4
3
2
1
0
–
–
–
–
–
SCLKSEL[2:0]
CTRL
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Table 7-1.
System clock selection.
SCLKSEL[2:0]
Group Configuration
Description
000
RC2MHZ
2MHz internal oscillator
001
RC32MHZ
32MHz internal oscillator
010
RC32KHZ
32.768kHz internal oscillator
011
XOSC
External oscillator or clock
100
PLL
Phase locked loop
101
—
Reserved
110
—
Reserved
111
—
Reserved
Bit
7
6
5
4
3
2
1
0
–
PSADIV[4:0]
PSBCDIV
PSCTRL
Read/Write
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0