Rainbow Electronics ATmega128L - Manual (Page 36)

36
ATmega128(L)
2467B–09/01
Notes: 1. These options should only be used when not operating close to the maximum fre-
quency of the device, and only if frequency stability at start-up is not important for the
application. These options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure fre-
quency stability at start-up. They can also be used with crystals when not operating
close to the maximum frequency of the device, and if frequency stability at start-up is
not important for the application.
Low-frequency Crystal
Oscillator To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency
crystal oscillator must be selected by setting the CKSEL fuses to “1001”. The crystal
should be connected as shown in
Figure 18. By programming the CKOPT fuse, the user
can enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for
external capacitors. The internal capacitors have a nominal value of 36 pF. Refer to the
32 kHz Crystal Oscillator application note for details on oscillator operation and how to
choose appropriate values for C1 and C2.
When this oscillator is selected, start-up times are determined by the SUT fuses as
shown in
Table 10.
Table 9. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
0
00 258 CK (1) 4 ms Ceramic resonator, fast
rising power
0
01 258 CK (1) 64 ms Ceramic resonator,
slowly rising power
0
10 1K CK (2) – Ceramic resonator,
BOD enabled
0
11 1K CK (2) 4 ms Ceramic resonator, fast
rising power
1
00 1K CK (2) 64 ms Ceramic resonator,
slowly rising power
1
01 16K CK - Crystal oscillator, BOD
enabled
1
10 16K CK 4 ms Crystal oscillator, fast
rising power
1
11 16K CK 64 ms Crystal oscillator,
slowly rising power
Table 10. Start-up Times for the low Frequency Crystal Oscillator Clock Selection
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
00 1K CK (1) 4 ms Fast rising power or BOD enabled
01 1K CK (1) 64 ms Slowly rising power
10 32K CK 64 ms Stable frequency at start-up
11 Reserved