ATMEGA328P-20AU Datasheet, PDF (33/420 Pages) ATMEL Corporation – 8-bit Atmel Microcontroller with 4/8/16K Bytes In-System Programmable Flash

ATmega48P/88P/168P
9.5 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.
ATmega48P/88P/168P oscillator is optimized for very low power consumption, and thus when
selecting crystals, see Table 9-7 on page 33 for maximum ESR recommendations on 6.5 pF,
9.0 pF and 12.5 pF crystals
Table 9-7. Maximum ESR Recommendation for 32.768 kHz Crystal
Crystal CL (pF)
6.5
9.0
12.5
Max ESR [kΩ]⁽¹⁾
75
65
30
Note: 1. Maximum ESR is typical value based on characterization
Table 9-8. Capacitance for the Low-frequency Crystal Oscillator
Device
32 kHz Osc.
Type
Cap
(Xtal1/Tosc1)
Cap
(Xtal2/Tosc2)
ATmega48P/88P/169P
System Osc.
Timer Osc.
18 pF
18 pF
8 pF
8 pF
The Low-frequency Crystal Oscillator provides an internal load capacitance of typical 6 pF at
each TOSC pin. The external capacitance (C) needed at each TOSC pin can be calculated by
using:
C = 2 ⋅ CL – C_s
where CL is the load capacitance for a 32.768 kHz crystal specified by the crystal vendor and C_S
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 30.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111”, as shown in Table 9-10. Start-up times are determined by the SUT Fuses as shown in
Table 9-9.
Table 9-9.
SUT[1:0]
00
01
10
11
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Additional Delay from Reset (V_CC = 5.0V) Recommended Usage
4 CK
Fast rising power or BOD enabled
4 CK + 4.1 ms
Slowly rising power
4 CK + 65 ms
Stable frequency at start-up
Reserved
33
8025M–AVR–6/11