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Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
5.10.1 Clock Prescaler Register – CLKPR
Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is only updated when the
other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by hardware four cycles after it is written or when
CLKPS bits are written. Rewriting the CLKPCE bit within this time-out period does neither extend the time-out period, nor clear
the CLKPCE bit.
Bits 3..0 – CLKPS3..0: Clock Prescaler Select Bits 3 - 0
These bits define the division factor between the selected clock source and the internal system clock. These bits can be written
run-time to vary the clock frequency to suit the application requirements. As the divider divides the master clock input to the
MCU, the speed of all synchronous peripherals is reduced when a division factor is used. The division factors are given in
TableThe CKDIV8 fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the CLKPS bits will be reset to
“0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a division factor of 8 at start up. This feature should be
used if the selected clock source has a higher frequency than the maximum frequency of the device at the present operating
conditions. Note that any value can be written to the CLKPS bits regardless of the CKDIV8 fuse setting. The application
software must ensure that a sufficient division factor is chosen if the selected clock source has a higher frequency than the
maximum frequency of the device at the present operating conditions. The device is shipped with the CKDIV8 fuse
programmed.
Bit
7
654
3
2
1
0
CLKPCE
–
CLKPS3
CLKPS2
CLKPS1
CLKPS0
CLKPR
Read/Write
R/W
R
R/W
Initial Value
0
See Bit Description
Table 5-10. Clock Prescaler Select
CLKPS3
CLKPS2
CLKPS1
CLKPS0
Clock Division Factor
0
1
0
1
2
0
1
0
4
0
1
8
0
1
0
16
0
1
0
1
32
0
1
0
64
0
1
128
1
0
256
1
0
1
Reserved
1
0
1
0
Reserved
1
0
1
Reserved
1
0
Reserved
1
0
1
Reserved
1
0
Reserved
1
Reserved