Datasheet

46
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
The capacitance (Ce + Ci) needed at each XTAL/TOSC pin can be calculated by using:
where:
Ce - is optional external capacitors as described in Figure 10-3 on page 46.
Ci - is the pin capacitance in Table 10-7 on page 45.
CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.
C
S
- is the total stray capacitance for one XTAL/TOSC pin.
Crystals specifying load capacitance (CL) higher than the ones given in the Table 10-7 on page
45, require external capacitors applied as described in Figure 10-3 on page 46.
Figure 10-3. Crystal Oscillator Connections
To find suitable load capacitance for a 32.768kHz crysal, please consult the crystal datasheet.
When this oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0
as shown in Table 10-8.
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
Table 10-8. Start-up times for the low frequency crystal oscillator clock selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) CKSEL0 SUT1:0
BOD enabled 1K CK 14CK
(1)
000
Fast rising power 1K CK 14CK + 4.1ms
(1)
001
Slowly rising power 1K CK 14CK + 65ms
(1)
010
Reserved 0 11
BOD enabled 32K CK 14CK 1 00
Fast rising power 32K CK 14CK + 4.1ms 1 01
Slowly rising power 32K CK 14CK + 65ms 1 10
Reserved 1 11
Ce Ci+ 2 CL C
s
–=
C
e
C
i
C
e
C
i
C
s
C
s
TOSC2
TOSC1
X1