ATmega325P datasheet(34/364 Pages) ATMEL

34 / 364 page

8023F–AVR–07/09

ATmega325P/3250P

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in

Table 8-9 on page 34.

Note:

1. The device is shipped with this option selected

8.7

External Clock

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

3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

Figure 3. External Clock Drive Configuration

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

Table 2.

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

Table 8-9.

Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

CC = 5.0V)

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms(1)

Reserved

Table 1. Crystal Oscillator Clock Frequency

CKSEL3..0

Frequency Range

0000

0 - 16 MHz

Table 2. Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

CC = 5.0V)

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4.1 ms

Fast rising power

6 CK

14CK + 65 ms

Slowly rising power

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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