34
8011Q–AVR–02/2013
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.
6.5
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 6-7.
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, see
Table 6-8 onTable 6-8.
Capasitance for Low-frequency Oscillator.
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
Table 6-6.
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
SUT1..0
Crystal CL (pF)
Max ESR [k
9.0
65
12.5
30
Device
32 kHz Osc. Type
Cap(Xtal1/Tosc1)
Cap(Xtal2/Tosc2)
ATmega164P/324P/644P
System Osc.
18 pF
8 pF
Timer Osc.
6 pF