ATMEGA640 datasheet(47/449 Pages) ATMEL

47 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

System Clock Prescaler

The ATmega640/1280/1281/2560/2561 has a system clock prescaler, and the system

clock can be divided by setting the “CLKPR – Clock Prescale Register” on page 48. This

feature can be used to decrease the system clock frequency and the power consump-

tion when the requirement for processing power is low. This can be used with all clock

source options, and it will affect the clock frequency of the CPU and all synchronous

peripherals. clk

I/O, clkADC, clkCPU, and clkFLASH are divided by a factor as shown in Table

23.

When switching between prescaler settings, the System Clock Prescaler ensures that

no glitches occurs in the clock system. It also ensures that no intermediate frequency is

higher than neither the clock frequency corresponding to the previous setting, nor the

clock frequency corresponding to the new setting.

The ripple counter that implements the prescaler runs at the frequency of the undivided

clock, which may be faster than the CPU's clock frequency. Hence, it is not possible to

determine the state of the prescaler - even if it were readable, and the exact time it takes

to switch from one clock division to the other cannot be exactly predicted. From the time

the CLKPS values are written, it takes between T1 + T2 and T1 + 2 * T2 before the new

clock frequency is active. In this interval, 2 active clock edges are produced. Here, T1 is

the previous clock period, and T2 is the period corresponding to the new prescaler

setting.

To avoid unintentional changes of clock frequency, a special write procedure must be

followed to change the CLKPS bits:

Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits

in CLKPR to zero.

Within four cycles, write the desired value to CLKPS while writing a zero to

CLKPCE.

Interrupts must be disabled when changing prescaler setting to make sure the write pro-

cedure is not interrupted.

ATMEGA640 Datasheet(PDF) 47 Page - ATMEL Corporation