ATmega48PA/88PA/168PA/328P
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.
ATmega48PA/88PA/168PA/328P oscillator is optimized for very low power consumption, and
thus when selecting crystals, see Table 8-7 on page 32 for maximum ESR recommendations on
6.5 pF, 9.0 pF and 12.5 pF crystals
Table 8-7.
Maximum ESR Recommendation for 32.768 kHz Crystal Oscillator
Crystal CL (pF)
Max ESR [kΩ](1)
6.5
9.0
75
65
30
12.5
Note:
1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance at each TOSC pin
as specified in the Table 8-8 on page 32.
Table 8-8.
Capacitance for Low-Frequency Crystal Oscillator
Device
32 kHz Osc. Type
Cap (Xtal1/Tosc1)
18 pF
Cap (Xtal2/Tosc2)
ATmega48PA/88PA/168PA/328P System Osc.
Timer Osc.
8 pF
8 pF
18 pF
The external capacitance (C) needed at each TOSC pin can be calculated by using:
C = 2 ⋅ CL – Cs
where CL is the load capacitance for a 32.768 kHz crystal specified by the crystal vendor and CS
is the total stray capacitance for one TOSC pin.
Crystals specifying load capacitance (CL) higher than the ones given in the Table 8-8 on page
32, require external capacitors applied as described in Figure 8-2 on page 29.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111”, as shown in Table 8-10. Start-up times are determined by the SUT Fuses as shown in
Table 8-9.
Table 8-9.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
00
Additional Delay from Reset (VCC = 5.0V) Recommended Usage
4 CK
Fast rising power or BOD enabled
Slowly rising power
01
4 CK + 4.1 ms
4 CK + 65 ms
10
Stable frequency at start-up
11
Reserved
32
8161D–AVR–10/09
ATMEL [ ATMEL ]
