ATMEGA3290 datasheet(30/375 Pages) ATMEL

30 / 375 page

ATmega329/3290/649/6490

2552H–AVR–11/06

External Clock

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

Figure 14. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to “0000”.

Figure 14. External Clock Drive Configuration

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

shown in Table 12.

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

clock frequency 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.

Note that the System Clock Prescaler can be used to implement run-time changes of

the internal clock frequency while still ensuring stable operation. Refer to “System Clock

Prescaler” on page 31 for details.

Table 11. Crystal Oscillator Clock Frequency

CKSEL3..0

Frequency Range

0000

0 - 16 MHz

Table 12. 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

ATMEGA3290 Datasheet(PDF) 30 Page - ATMEL Corporation

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