ATmega16 datasheet(28/357 Pages) ATMEL

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2466S–AVR–05/09

ATmega16(L)

Low-frequency

Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal

Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con-

nected as shown in Figure 12. By programming the CKOPT Fuse, the user can enable internal

capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The inter-

nal capacitors have a nominal value of 36 pF.

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

Table 6.

Note:

1. These options should only be used if frequency stability at start-up is not important for the

application.

External RC

Oscillator

For timing insensitive applications, the external RC configuration shown in Figure 13 can be

used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22

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

XTAL1 and GND, thereby removing the need for an external capacitor. For more information on

Oscillator operation and details on how to choose R and C, refer to the External RC Oscillator

application note.

Figure 13. External RC Configuration

The Oscillator can operate in four different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..0 as shown in Table 7.

Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

Recommended Usage

1K CK(1)

4.1 ms

Fast rising power or BOD enabled

1K CK(1)

65 ms

Slowly rising power

32K CK

65 ms

Stable frequency at start-up

Reserved

Table 7. External RC Oscillator Operating Modes

CKSEL3..0

Frequency Range (MHz)

0101

0.1

≤ 0.9

0110

0.9 - 3.0

0111

3.0 - 8.0

1000

8.0 - 12.0

XTAL2

XTAL1

GND

VCC

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