Rainbow Electronics ATmega128L - Manual (Page 37)

37
ATmega128(L)
2467B–09/01
Note: 1. These options should only be used if frequency stability at start-up is not important
for the application.
External RC Oscillator For timing insensitive applications, the external RC configuration shown in Figure 19
can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should
be at least 22 pF. By programming the CKOPT fuse, the user can enable an internal
36 pF capacitor between XTAL1 and GND, thereby removing the need for an external
capacitor. For more information on oscillator operation and details on how to choose R
and C, refer to the External RC Oscillator Application Note.
Figure 19. External RC Configuration
The oscillator can operate in four different modes, each optimized for a specific fre-
quency range. The operating mode is selected by the fuses CKSEL3..0 as shown in
Table 11.
When this oscillator is selected, start-up times are determined by the SUT fuses as
shown in
Table 12.
Note: 1. This option should not be used when operating close to the maximum frequency of
the device.
Table 11. External RC Oscillator Operating Modes
CKSEL3..0 Frequency Range (MHz)
0101 - 0.9
0110 0.9 - 3.0
0111 3.0 - 8.0
1000 8.0 - 12.0
Table 12. Start-Up Times for the External RC 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 18 CK - BOD enabled
01 18 CK 4 ms Fast rising power
10 18 CK 64 ms Slowly rising power
11 6 CK (1) 4 ms Fast rising power or BOD enabled
XTAL2
XTAL1
GND
C
R
V
CC
NC