Datasheet
For the Crystall Oscillator connections refer to Low Power Crystal Oscillator.
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 SUT[1:0]
Ceramic resonator, fast rising power 258 CK 14CK + 4.1ms
(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 + 4.1ms
(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 + 4.1ms 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
10.5. Low Frequency Crystal Oscillator
The Low-frequency Crystal Oscillator is optimized for use with a 32.768kHz watch crystal. When selecting
crystals, load capacitance and crystal’s Equivalent Series Resistance (ESR) must be taken into
consideration. Both values are specified by the crystal vendor. The oscillator is optimized for very low
power consumption, and thus when selecting crystals, consider the Maximum ESR Recommendations:
Table 10-7. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL [pF] Max. ESR [kΩ]
9.0 65
12.5 30
The Low-frequency Crystal Oscillator provides an internal load capacitance at each TOSC pin:
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
47