ATmega169P datasheet(36/393 Pages) ATMEL

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8018N–AVR–08/09

ATmega169P

8.7

External Clock

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

8-3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

Figure 8-3.

External Clock Drive Configuration

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

Table 8-11.

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

37 for details.

Table 8-10.

Crystal Oscillator Clock Frequency

CKSEL3..0

Frequency Range

0000

0 - 16 MHz

Table 8-11.

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

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