35
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
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.
9.5
Low Frequency Crystal Oscillator
The Low-frequency Crystal Oscillator is optimized for use with a 32.768kHz watch crystal. When
selecting crystals, load capasitance and crystal’s Equivalent Series Resistance, ESR must be
ta ken into cons ideration. Both values ar e specified by the crystal vendor. Atmel
ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P oscillator is optimized for very low
ESR recommendations on 9pF and 12.5pF crystals.
Note:
1. Maximum ESR is typical value based on characterization.
The Low-frequency Crystal Oscillator provides an internal load capacitance, see
Table on pageThe capacitance (Ce + Ci) needed at each TOSC pin can be calculated by using:
where:
CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor.
C
S - is the total stray capacitance for one TOSC pin.
Crystals specifying load capacitance (CL) higher than the ones given in the
Table 9-8 on pageFigure 9-3.
Crystal oscillator connections.
Table 9-7.
Maximum ESR Recommendation for 32.768kHz Watch Crystal.
Crystal CL [pF]
Max ESR [k
9.0
65
12.5
30
Table 9-8.
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.
18pF
8pF
Timer osc.
6pF
Ce
Ci
+
2 CL
C
s
–
=