ATmega128 datasheet(43/386 Pages) ATMEL

43 / 386 page

2467S–AVR–07/09

ATmega128

to the register gives the highest available frequency. The calibrated Oscillator is used to time

EEPROM and Flash access. If EEPROM or Flash is written, do not calibrate to more than 10%

above the nominal frequency. Otherwise, the EEPROM or Flash write may fail. Note that the

Oscillator is intended for calibration to 1.0, 2.0, 4.0, or 8.0 MHz. Tuning to other values is not

guaranteed, as indicated in Table 15.

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

21. To run the device on an external clock, the CKSEL fuses must be programmed to “0000”. By

programming the CKOPT fuse, the user can enable an internal 36 pF capacitor between XTAL1

and GND.

Figure 21. External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT fuses as shown in

Table 16.

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from

one clock cycle to the next can lead to unpredictable behavior. It is required to ensure that the

MCU is kept in Reset during such changes in the clock frequency.

Table 15. Internal RC Oscillator Frequency Range.

OSCCAL Value

Min Frequency in Percentage of

Nominal Frequency (%)

Max Frequency in Percentage of

Nominal Frequency (%)

$00

100

$7F

150

$FF

100

200

Table 16. Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (VCC = 5.0V)

Recommended Usage

6 CK

–

BOD enabled

6 CK

4.1 ms

Fast rising power

6 CK

65 ms

Slowly rising power

Reserved

EXTERNAL

CLOCK

SIGNAL

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