Datasheet

45
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
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.
10.6 Low Frequency Crystal Oscillator
The device can utilize a 32.768kHz watch crystal as clock source by a dedicated Low Frequency
Crystal Oscillator. The crystal should be connected as shown in Figure 10-3 on page 46. When
this Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as
shown in Table 10-8 on page 46.
The Low-Frequency Crystal Oscillator provides an internal load capacitance, see Table 10-7 at
each XTAL/TOSC pin.
Table 10-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) CKSEL0 SUT1:0
Ceramic resonator, fast
rising power
258 CK 14CK + 4.1ms
(1)
000
Ceramic resonator, slowly
rising power
258 CK 14CK + 65ms
(1)
001
Ceramic resonator, BOD
enabled
1K CK 14CK
(2)
010
Ceramic resonator, fast
rising power
1K CK 14CK + 4.1ms
(2)
011
Ceramic resonator, slowly
rising power
1K CK 14CK + 65ms
(2)
100
Crystal Oscillator, BOD
enabled
16K CK 14CK 1 01
Crystal Oscillator, fast
rising power
16K CK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly
rising power
16K CK 14CK + 65ms 1 11
Table 10-7. Capacitance for Low frequency oscillator
Device 32kHz oscillator Cap (Xtal1/Tosc1) Cap (Xtal2/Tosc2)
ATmega640/1280/1281/2560/2561
System Osc. 18pF 8pF
Timer Osc. 6pF 6pF