ATmega16 datasheet(26/315 Pages) ATMEL

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ATmega16(L)

2466E–AVR–10/02

Low-frequency Crystal

Oscillator

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

quency Crystal Oscillator must be selected by setting the CKSEL fuses to “1001”. The

crystal should be connected 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 internal capacitors have a nominal value of 36 pF.

Refer to the 32 kHz Crystal Oscillator application note for details on Oscillator operation

and how to choose appropriate values for C1 and C2.

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 fre-

quency 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

(VCC = 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

XTAL2

XTAL1

GND

VCC

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