Datasheet
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
taken into consideration. Both values are specified by the crystal vendor. Atmel
ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P oscillator is optimized for very low
power consumption, and thus when selecting crystals, see Table 9-7 on page 35 for maximum
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 page
35 at each TOSC pin.
The capacitance (Ce + Ci) needed at each TOSC pin can be calculated by using:
where:
Ce - is optional external capacitors as described in Figure 9-2 on page 32
Ci - is is the pin capacitance in Table 9-8 on page 35
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 page
35, require external capacitors applied as described in Figure 9-2 on page 32.
Figure 9-3. Crystal oscillator connections.
Crystals specifying load capacitance (CL) higher than listed in Table 9-8 on page 35, require
external capacitors applied as described in Figure 9-2 on page 32.
Table 9-7. Maximum ESR Recommendation for 32.768kHz Watch Crystal.
Crystal CL [pF] Max ESR [k]
(1)
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 6pF
Ce Ci+ 2 CL C
s
–=