Rainbow Electronics ATmega128L - Manual (Page 35)

35
ATmega128(L)
2467B–09/01
For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and 16
MHz with CKOPT programmed. 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 environ-
ment. Some initial guidelines for choosing capacitors for use with crystals are given in
Table 8. For ceramic resonators, the capacitor values given by the manufacturer should
be used. For more information on how to choose capacitors and other details on oscilla-
tor operation, refer to the Multi-purpose Oscillator application note.
Figure 18. Crystal Oscillator Connections
The oscillator can operate in three different modes, each optimized for a specific fre-
quency range. The operating mode is selected by the fuses CKSEL3..1 as shown in
Table 8.
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. 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 9.
Table 8. Crystal Oscillator Operating Modes
CKOPT CKSEL3..1
Frequency Range (1)
(MHz)
Recommended Range for Capacitors
C1 and C2 for Use with Crystals
1 101 (2) 0.4 - 0.9 –
1 110 0.9 - 3.0 12 pF - 22 pF
1 111 3.0 - 8.0 12 pF - 22 pF
0 101, 110, 111 1.0 - 12 pF - 22 pF
XTAL2
XTAL1
GND
C2
C1