Datasheet

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 SUT[1:0]
Ceramic resonator, fast rising power 258 CK 14CK + 4ms
(1)
0 00
Ceramic resonator, slowly rising
power
258 CK 14CK + 65ms
(1)
0 01
Ceramic resonator, BOD enabled 1K CK 14CK
(2)
0 10
Ceramic resonator, fast rising power 1K CK 14CK + 4ms
(2)
0 11
Ceramic resonator, slowly rising
power
1K CK 14CK + 65ms
(2)
1 00
Crystal Oscillator, BOD enabled 16K CK 14CK 1 01
Crystal Oscillator, fast rising power 16K CK 14CK + 4ms 1 10
Crystal Oscillator, slowly rising power 16K CK 14CK + 65ms 1 11
Note:
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 stability at start-up is not important for the application.
Related Links
Clock Source Connections on page 44
10.4. Full Swing Crystal Oscillator
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 for the
Low Power Crystal Oscillator. Note that the Full Swing Crystal Oscillator will only operate for
V
CC
=2.7-5.5V.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 10-6. The crystal
should be connected as described in Clock Source Connections”.
The operating mode is selected based on the fuses CKSEL[3:1] as shown in the table:
Table 10-5. Full Swing Crystal Oscillator operating modes
Frequency Range
(1)
[MHz] CKSEL[3:1] Recommended Range for Capacitors C1 and C2 [pF]
0.4 - 20 011 12 - 22
Note:
1. If the crystal frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8 Fuse
can be programmed 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.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
46