ATmega32A datasheet(28/352 Pages) ATMEL

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8155B–AVR–07/09

ATmega32A

The CKSEL0 Fuse together with the SUT1:0 fuses select the start-up times as shown in Table 8-

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.

8.5

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 8-2. 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 8-5.

Table 8-4.

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

(VCC = 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

010

1K CK(2)

–

Ceramic resonator, BOD

enabled

011

1K CK(2)

4.1 ms

Ceramic resonator, fast rising

power

100

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 8-5.

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

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