Datasheet

13.12.2 Clock Prescaler Register

Name: CLKPR

Offset: 0x61

Reset: Refer to the bit description

Property: -

Bit 7 6 5 4 3 2 1 0

CLKPCE CLKPS[3:0]

Access

R/W R/W R/W R/W R/W

Reset 0 x x x x

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

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 the table below.

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 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.

Table 13-17. Clock Prescaler Select

CLKPS[3:0] Clock Division Factor

0000 1

0001 2

0010 4

0011 8

0100 16

0101 32

0110 64

0111 128

1000 256

ATmega48PA/88PA/168PA

System Clock and Clock Options

Datasheet Complete

DS40002011A-page 66