Rainbow-electronics ATmega128L Manuel d'utilisateur télécharger pdf (Page 36)

ATmega128(L)

2467B–09/01

Notes: 1. These options should only be used when not operating close to the maximum fre-

quency 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 fre-

quency stability at start-up. They can also be used with crystals when not operating

close to the maximum frequency 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 connected as shown in

Figure 18. 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 10.

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

= 5.0V) Recommended Usage

00 258 CK

(1)

4 ms Ceramic resonator, fast

rising power

01 258 CK

(1)

64 ms Ceramic resonator,

slowly rising power

10 1K CK

(2)

– Ceramic resonator,

BOD enabled

11 1K CK

(2)

4 ms Ceramic resonator, fast

rising power

00 1K CK

(2)

64 ms Ceramic resonator,

slowly rising power

01 16K CK - Crystal oscillator, BOD

enabled

10 16K CK 4 ms Crystal oscillator, fast

rising power

11 16K CK 64 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

= 5.0V) Recommended Usage

00 1K CK

(1)

4 ms Fast rising power or BOD enabled

01 1K CK

(1)

64 ms Slowly rising power

10 32K CK 64 ms Stable frequency at start-up

11 Reserved

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 330 331

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Rainbow-electronics ATmega128L Manuel d'utilisateur Page 36

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