
34
7674F–AVR–09/09
ATmega164P/324P/644P
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.
7.5
Low Frequency Crystal Oscillator
The Low-frequency Crystal Oscillator is optimized for use with a 32.768 kHz watch crystal.
When selecting crystals, load capasitance and crystal’s Equivalent Series Resistance, ESR
must be taken into consideration. Both values are specified by the crystal vendor.
ATmega164P/324P/644P oscillator is optimized for very low power consumption, and thus when
12.5 pF crystals
Table 7-7.
Maximum ESR Recommendation for 32.768 kHz Watch Crystal
Note:
1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance of typical 8.0 pF.
Crystals with recommended 8.0 pF load capacitance can be without external capacitors as
Table 7-6.
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
(VCC = 5.0V)
CKSEL0
SUT1..0
Ceramic resonator, fast
rising power
258 CK
000
Ceramic resonator, slowly
rising power
258 CK
001
Ceramic resonator, BOD
enabled
1K CK
010
Ceramic resonator, fast
rising power
1K CK
011
Ceramic resonator, slowly
rising power
1K CK
100
Crystal Oscillator, BOD
enabled
16K CK
14CK
1
01
Crystal Oscillator, fast
rising power
16K CK
14CK + 4.1 ms
1
10
Crystal Oscillator, slowly
rising power
16K CK
14CK + 65 ms
1
11
Crystal CL (pF)
Max ESR [k
9.0
65
12.5
30