ATMEGA6450V-8AJ 查看數據表（PDF） - Atmel Corporation

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ATMEGA6450V-8AJ

8-bit Microcontroller with In-System Programmable Flash

Atmel Corporation 

Figure 14. External Clock Drive Configuration

NC

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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

shown in Table 12.

Table 11. Crystal Oscillator Clock Frequency

CKSEL3..0

0000

Frequency Range

0 - 16 MHz

Table 12. Start-up Times for the External Clock Selection

SUT1..0

00

Start-up Time from Power-

down and Power-save

6 CK

Additional Delay from

Reset (VCC = 5.0V)

14CK

Recommended Usage

BOD enabled

01

6 CK

14CK + 4.1 ms

Fast rising power

10

6 CK

14CK + 65 ms

Slowly rising power

11

Reserved

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.

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 output also during reset and the normal operation of I/O pin will be overrid-

den 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.

Timer/Counter Oscillator ATmega325/3250/645/6450 share the Timer/Counter Oscillator Pins (TOSC1 and

TOSC2) with XTAL1 and XTAL2. This means that the Timer/Counter Oscillator can only

be used when the calibrated internal RC Oscillator is selected as system clock source.

30 ATmega325/3250/645/6450

2570A–AVR–09/04

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