ATMEGA3250PV_14 Datasheet, PDF (31/364 Pages) ATMEL Corporation – High Endurance Non-volatile Memory segments

ATmega325P/3250P
8.4 Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
figured for use as an On-chip Oscillator, as shown in Figure 8-2. Either a quartz crystal or a
ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal 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 crystals are given in Table 8-3. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Figure 8-2. Crystal Oscillator Connections
C2
XTAL2
C1
XTAL1
GND
8023F–AVR–07/09
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 8-3.
Table 8-3. Crystal Oscillator Operating Modes
CKSEL3..1
100⁽¹⁾
Frequency Range
(MHz)
0.4 - 0.9
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
–
101
0.9 - 3.0
12 - 22
110
3.0 - 8.0
12 - 22
111
8.0 -
12 - 22
Note: 1. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
8-4.
Table 8-4.
CKSEL0
0
0
0
Start-up Times for the Crystal Oscillator Clock Selection
SUT1..0
00
Start-up Time from
Power-down and
Power-save
258 CK⁽¹⁾
Additional Delay from
Reset
(V_CC = 5.0V)
14CK + 4.1 ms
Recommended Usage
Ceramic resonator, fast
rising power
01
258 CK⁽¹⁾
14CK + 65 ms
Ceramic resonator,
slowly rising power
10
1K CK⁽²⁾
14CK
Ceramic resonator,
BOD enabled
31