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ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
8.11.3
CLKPR – Clock Prescaler Register
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 simultaniosly 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 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 eight at
start up. This feature should be used if the selected clock source has a higher frequency than the maximum fre-
quency 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 selcted 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 8-10.
Clock prescaler select.
CLKPS3
CLKPS2
CLKPS1
CLKPS0
Clock division factor
00
1
00
01
2
00
10
4
00
11
8
01
00
16
01
32
01
10
64
01
11
128
10
00
256
10
01
Reserved
10
Reserved
10
11
Reserved
11
00
Reserved
11
01
Reserved
11
10
Reserved
11
Reserved