Data Sheet

33
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
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 capacitance and crystal’s Equivalent Series Resistance, ESR must be taken into consideration.
Both values are specified by the crystal vendor. ATmega48A/PA/88A/PA/168A/PA/328/P oscillator is optimized
for very low power consumption, and thus when selecting crystals, see Table for maximum ESR
recommendations on 6.5pF, 9.0pF 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 9-8 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 29
̶ Ci - is the pin capacitance in Table 9-8
̶ CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor
̶ CS - is the total stray capacitance for one TOSC pin.
Crystals specifying load capacitance (CL) higher than 6 pF, require external capacitors applied as described in
Figure 9-2 on page 29.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or “0111”, as
shown in Table 9-10 on page 34. Start-up times are determined by the SUT Fuses as shown in Table 9-9.
Table 9-7. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL (pF) Max ESR [k]
(1)
6.5 75
9.0 65
12.5 30
Table 9-8. Capacitance for Low-frequency Oscillator
Device 32kHz Osc. Type Cap(Xtal1/Tosc1) Cap(Xtal2/Tosc2)
ATmega48A/PA/88A/PA/168A/PA/3
28/P
System Osc. 18pF 8pF
Timer Osc. 18pF 8pF
Table 9-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1...0 Additional Delay from Reset (V
CC
= 5.0V) Recommended Usage
00 4 CK Fast rising power or BOD enabled
01 4 CK + 4.1ms Slowly rising power
10 4 CK + 65ms Stable frequency at start-up
11 Reserved
C 2 CL C
s
–=