ATMEGA3290P datasheet(34/360 Pages) ATMEL

34 / 360 page

8021A–AVR–12/06

ATmega329P/3290P

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

Table 7-12 on page 34.

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.

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

35 for details.

7.8

Clock Output Buffer

When the CKOUT Fuse is programmed, the system Clock will be output on CLKO. This mode is

suitable when the chip clock is used to drive other circuits on the system. The clock will be out-

put also during reset and the normal operation of I/O pin will be overridden when the fuse is

programmed. Any clock source, including internal RC Oscillator, can be selected when CLKO

serves as clock output. If the System Clock Prescaler is used, it is the divided system clock that

is output when the CKOUT Fuse is programmed.

7.9

Timer/Counter Oscillator

ATmega329P/3290P uses the same crystal oscillator for Low-frequency Oscillator and

Timer/Counter Oscillator. See ”Low-frequency Crystal Oscillator” on page 31 for details on the

oscillator and crystal requirements.

ATmega329P/3290P share the Timer/Counter Oscillator Pins (TOSC1 and TOSC2) with XTAL1

and XTAL2. When using the Timer/Counter Oscillator, the system clock needs to be four times

the oscillator frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only

be used when the Calibrated Internal RC Oscillator is selected as system clock source.

Applying an external clock source to TOSC1 can be done if EXTCLK in the ASSR Register is

written to logic one. See ”Asynchronous Operation of Timer/Counter2” on page 151 for further

description on selecting external clock as input instead of a 32.768 kHz watch crystal.

Table 7-11.

Crystal Oscillator Clock Frequency

CKSEL3..0

Frequency Range

0000

0 - 16 MHz

Table 7-12.

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

14CK

BOD enabled

6 CK

14CK + 4.1 ms

Fast rising power

6 CK

14CK + 65 ms

Slowly rising power

Reserved

ATMEGA3290P Datasheet(PDF) 34 Page - ATMEL Corporation

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