Datasheet

35
8272A–AVR–01/10
164A/164PA/324A/324PA/644A/644PA/1284/1284P
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.
8.5 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.
ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P oscillator is optimized for very low
power consumption, and thus when selecting crystals, see Table 8-7 on page 35 for maximum
ESR recommendations on 9 pF and 12.5 pF crystals.
Note: 1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table on page
35 at each TOSC pin.
The capacitance (Ce + Ci) needed at each TOSC pin can be calculated by using:
where:
Ceramic resonator, fast
rising power
1K CK 14CK + 4.1 ms
(2)
011
Ceramic resonator, slowly
rising power
1K CK 14CK + 65 ms
(2)
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 8-6. 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
Table 8-7. Maximum ESR Recommendation for 32.768 kHz Watch Crystal
Crystal CL (pF) Max ESR [kΩ]
(1)
9.0 65
12.5 30
Table 8-8. Capacitance for Low-frequency Oscillator
Device 32kHz Osc. Type Cap (Xtal1/Tosc1) Cap (Xtal2/Tosc2)
ATmega164A/164PA/324A/324PA/
644A/644PA/1284/1284P
System Osc. 18 pF 8 pF
Timer Osc. 6 pF 6 pF
Ce Ci+ 2 CL⋅ C
s
–=