ATmega8(L)
External RC Oscillator
For timing insensitive applications, the external RC configuration shown in Figure 12
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 12. External RC Configuration
V
CC
NC
XTAL2
XTAL1
GND
R
C
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 7.
Table 7. External RC Oscillator Operating Modes
CKSEL3..0
0101
Frequency Range (MHz)
≤ 0.9
0110
0.9 - 3.0
0111
3.0 - 8.0
1000
8.0 - 12.0
When this Oscillator is selected, start-up times are determined by the SUT Fuses as
shown in Table 8.
Table 8. Start-up Times for the External RC Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
SUT1..0
00
(VCC = 5.0V)
Recommended Usage
BOD enabled
18 CK
18 CK
18 CK
6 CK(1)
–
01
4.1 ms
65 ms
4.1 ms
Fast rising power
10
Slowly rising power
11
Fast rising power or BOD enabled
Note:
1. This option should not be used when operating close to the maximum frequency of
the device.
27
2486M–AVR–12/03
ATMEL [ ATMEL ]
