Datasheet

42
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
Notes: 1. This is the recommended CKSEL settings for the different frequency ranges.
2. This option should not be used with crystals, only with ceramic resonators.
3. If 8MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8 Fuse can be pro-
grammed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets
the frequency specification of the device.
4. Maximum frequency when using ceramic oscillator is 10MHz.
The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in Table 10-4.
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 frequency of the device, and if frequency sta-
bility at start-up is not important for the application.
10.5 Full Swing Crystal Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use
as an On-chip Oscillator, as shown in Figure 10-2 on page 41. Either a quartz crystal or a ceramic resonator may
be used.
This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for driving
other clock inputs and in noisy environments. The current consumption is higher than the “Low Power Crystal
Oscillator” on page 41. Note that the Full Swing Crystal Oscillator will only operate for V
CC
= 2.7 - 5.5 volts.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends
on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environ-
ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 10-6 on page 43. For
ceramic resonators, the capacitor values given by the manufacturer should be used.
The operating mode is selected by the fuses CKSEL3:1 as shown in Table 10-5 on page 43.
Table 10-3. Low Power Crystal Oscillator Operating Modes
(3)
Frequency Range [MHz] CKSEL3:1
(1)
Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 0.9 100
(2)
–
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0
(4)
111 12 - 22
Table 10-4. Start-up Times for the Low Power 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 258CK 14CK + 4.1ms
(1)
000
Ceramic resonator, slowly rising power 258CK 14CK + 65ms
(1)
001
Ceramic resonator, BOD enabled 1KCK 14CK
(2)
010
Ceramic resonator, fast rising power 1KCK 14CK + 4.1ms
(2)
011
Ceramic resonator, slowly rising power 1KCK 14CK + 65ms
(2)
100
Crystal Oscillator, BOD enabled 16KCK 14CK 1 01
Crystal Oscillator, fast rising power 16KCK 14CK + 4.1ms 1 10
Crystal Oscillator, slowly rising power 16KCK 14CK + 65ms 1 11