Rainbow-electronics ATmega8515L Manual do Utilizador descarregar pdf (Página 34)

ATmega8515(L)

2512A–AVR–04/02

Notes: 1. Theseoptionsshould only beusedwhen not operating closetothemaximum fre-

quency of the device, and only iffrequency stability at start-up is not important for the

application. Theseoptions are not suitable forcrystals.

2. Theseoptions are intendedfor use withceramicresonators andwill ensure fre-

quency stability at start-up.Theycan also beusedwithcrystals when not operating

closetothemaximum frequency of the device, and iffrequency stability at start-up is

not important for theapplication.

Low-frequency Crystal

Oscillator

To usea32.768 kHz watch crystal as the clock source for the device, the Low-fre-

quency CrystalOscillator must be selectedbysetting the CKSEL Fuses to “1001”. The

crystalshould be connected asshowninFigure18.By programming the CKOPT Fuse,

theusercan enableinternalcapacitors on XTAL1andXTAL2, thereby removing the

needfor externalcapacitors.Theinternalcapacitors have a nominal value of36pF.

When thisOscillator isselected, start-up times are determinedbythe SUT Fuses as

showninTable 9.

Note: 1. Theseoptionsshould only beused iffrequency stability at start-up is not important

for theapplication.

Table 9. Start-up Timesfor the Low-frequency CrystalOscillatorClock Selection

SUT1..0

Start-up Time

from Power-down

Additional Delay from

Reset (V

= 5.0V) Recommended Usage

00 1KCK

(1)

4.1 msFast rising power orBOD

enabled

01 1KCK

(1)

65msSlowly rising power

10 32KCK 65msStable frequency at start-up

11 Reserved

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 222 223

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