ATmega128 datasheet(40/386 Pages) ATMEL

40 / 386 page

2467S–AVR–07/09

ATmega128

Notes:

1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

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 19. 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 10.

Note:

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

application.

Table 9. Start-up Times for the Crystal Oscillator Clock Selection

CKSEL0

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset (V

= 5.0V)

Recommended Usage

258 CK(1)

4.1 ms

Ceramic resonator, fast

rising power

258 CK(1)

65 ms

Ceramic resonator,

slowly rising power

1K CK(2)

–

Ceramic resonator,

BOD enabled

1K CK(2)

4.1 ms

Ceramic resonator, fast

rising power

1K CK(2)

65 ms

Ceramic resonator,

slowly rising power

16K CK

–

Crystal Oscillator, BOD

enabled

16K CK

4.1 ms

Crystal Oscillator, fast

rising power

16K CK

65 ms

Crystal Oscillator,

slowly rising power

Table 10. 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 (V

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

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