ATmega16U4 datasheet(38/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

• 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 – CLKPS[3..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 synchro-

nous peripherals is reduced when a division factor is used. The division factors are given in

Table 6-10.

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 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 operat-

ing 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.

CLK-

PCE

––

–CLKPS

CLKPS

CLKPR

Read/Write

R/W

Initial Value

See Bit Description

Table 6-10.

Clock Prescaler Select

CLKPS3

CLKPS2

CLKPS1

CLKPS0

Clock Division Factor

0000

0001

0010

0011

0100

0101

0110

0111

128

1000

256

1001

Reserved

1010

Reserved

1011

Reserved

1100

Reserved

1101

Reserved

1110

Reserved

1111

Reserved

ATmega16U4 Datasheet(PDF) 38 Page - ATMEL Corporation

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