ATMEGA48 datasheet(35/374 Pages) ATMEL

35 / 374 page

2545J–AVR–12/06

ATmega48/88/168

7.8

External Clock

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

7-4 on page 35. To run the device on an external clock, the CKSEL Fuses must be programmed

to “0000” (see Table 7-12).

Figure 7-4.

External Clock Drive Configuration

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

Table 7-13.

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

36 for details.

7.9

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

Table 7-12.

Crystal Oscillator Clock Frequency

Frequency

CKSEL3..0

0 - 20 MHz

0000

Table 7-13.

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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