Datasheet

29
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
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:
C = 2 CL – C
S
where:
● Ce - is optional external capacitors as described in Figure 9-2 on page 26
● 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 6pF, require external capacitors applied as described in
Figure 9-2 on page 26.
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 9-8. Capacitance for Low-frequency Oscillator
Device 32kHz Osc. Type Cap(Xtal1/Tosc1) Cap(Xtal2/Tosc2)
Atmel ATmega48PA/88PA/168PA
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
Table 9-10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3...0
Start-up Time from
Power-down and Power-save Recommended Usage
0100
(1)
1K CK
0101 32K CK Stable frequency at start-up
Note: 1. This option should only be used if frequency stability at start-up is not important for the application