ATmega16A datasheet(30/352 Pages) ATMEL

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8154B–AVR–07/09

ATmega16A

8.8

External Clock

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

8-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between

XTAL1 and GND.

Figure 8-4.

External Clock Drive Configuration

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

Table 8-10.

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.

8.9

Timer/Counter Oscillator

For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is

connected directly between the pins. No external capacitors are needed. The Oscillator is opti-

mized for use with a 32.768 kHz watch crystal. Applying an external clock source to TOSC1 is

not recommended.

Note:

The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency Oscillator

and the internal capacitors have the same nominal value of 36 pF.

Table 8-10.

Start-up Times for the External Clock Selection

SUT1:0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

Recommended Usage

6 CK

–

BOD enabled

6 CK

4.1 ms

Fast rising power

6 CK

65 ms

Slowly rising power

Reserved

EXTERNAL

CLOCK

SIGNAL

ATmega16A Datasheet(PDF) 30 Page - ATMEL Corporation

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