ATmega48 datasheet(33/340 Pages) ATMEL

33 / 340 page

7530H–AVR–02/09

ATmega48/88/168 Automotive

Figure 6-4.

External Clock Drive Configuration

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

Table 6-13.

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. If changes of more than 2% is

required, ensure that the MCU is kept in Reset during the changes.

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

34 for details.

6.9

Clock Output Buffer

The device can output the system clock on the CLKO pin. To enable the output, the CKOUT

Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other cir-

cuits on the system. The clock also will be output during reset, and the normal operation of I/O

pin will be overridden when the fuse is programmed. Any clock source, including the internal RC

Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is

used, it is the divided system clock that is output.

6.10

Timer/Counter Oscillator

The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or a exter-

nal clock source. The Timer/Counter Oscillator Pins (TOSC1 and TOSC2) are shared with

XTAL1 and XTAL2. This means that the Timer/Counter Oscillator can only be used when an

internal RC Oscillator is selected as system clock source. See Figure 6-2 on page 27 for crystal

connection.

Table 6-13.

Start-up Times for the External 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

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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