Datasheet

35
8284D–AVR–6/11
ATmega169A/PA/329A/PA/3290A/PA/649A/P/6490A/P
The Low-frequency Crystal Oscillator provides an internal load capacitance, seeTable 9-9 at
each TOSC pin.
Table 9-9. Capacitance for Low-frequency Oscillator.
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 33
Ci - is the pin capacitance in Table 9-10
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 9-9, require
external capacitors applied as described in Figure 9-2 on page 33.
To find suitable load capacitance for a 32.768kHz crystal, please consult the crystal datasheet.
The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111” as shown in Table 9-11 on page 35. Start-up times are determined by the SUT Fuses as
shown in Table 9-10.
Note: 1. This option should only be used if frequency stability at start-up is not important for the
application
9.7 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
9-3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Device 32kHz Osc. Type Cap(Xtal1/Tosc1) Cap(Xtal2/Tosc2)
ATmega169A/169PA/329A/329PA/3290A
/3290PA/649A/649P/6490A/6490P
System Osc. 16pF 6pF
Timer Osc. 16pF 6pF
Table 9-10. 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 9-11. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3...
0
Start-up Time from
Power-down and Power-save Recommended Usage
0110
(1)
1K CK
0111 32K CK Stable frequency at start-up
Ce Ci+ 2 CL⋅ C
s
–=