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ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
9.5
XTAL oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as
an On-chip Oscillator, as shown in
Figure 9-2. Either a quartz XTAL or a ceramic resonator may be used.
C1 and C2 should always be equal for both XTALs and resonators. The optimal value of the capacitors depends on
the XTAL or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environment.
Some initial guidelines for choosing capacitors for use with XTALs are given in
Table 9-5. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
Figure 9-2.
XTAL oscillator connections.
The Oscillator can operate in three different modes, each optimized for a specific frequency range. The operating
mode is selected by the fuses CKSEL3:1 as shown in
Table 9-5.
Notes:
1. This option should not be used with XTALs, only with ceramic resonators.
The CKSEL0 fuse together with the SUT1:0 fuses select the start-up times as shown in
Table 9-6.
Table 9-5.
XTAL oscillator operating modes.
CKSEL3:1
Frequency range [MHz]
Recommended range for capacitors C1 and
C2 for use with XTALs [pF]
0.4 - 0.9
–
101
0.9 - 3.0
12 - 22
110
3.0 - 8.0
12 - 22
111
8.0 -
12 - 22
Table 9-6.
Start-up times for the XTAL oscillator clock selection.
CKSEL0
SUT1:0
Start-up time from
power-down and
power-save
Additional delay
from reset
(VCC = 5.0V)
Recommended usage
0
00
14CK + 4.1ms
Ceramic resonator, fast
rising power
0
01
14CK + 65ms
Ceramic resonator, slowly
rising power
010
14CK
Ceramic resonator, BOD
enabled
011
14CK + 4.1ms
Ceramic resonator, fast
rising power
100
14CK + 65ms
Ceramic resonator, slowly
rising power
XTAL2 (TOSC2)
XTAL1 (TOSC1)
GND
C2
C1