
87
XMEGA A [MANUAL]
8077I–AVR–11/2012
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
Table 7-1 on page 87 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 configuration change protection mechanism. For details, refer to
SCLKSEL cannot be changed if the new clock source is not stable. The old clock can not be disabled until the clock
switching is completed.
Table 7-1.
System clock selection.
7.9.2
PSCTRL – Prescaler register
This register is protected by the configuration change protection mechanism. For details, refer to
“Configuration ChangeBit 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
765
43210
+0x00
–
SCLKSEL[2:0]
Read/Write
R
R/W
Initial Value
000
00000
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
543210
+0x01
–
PSADIV[4:0]
PSBCDIV
Read/Write
R
R/W
Initial Value
0
000000