ATMEGA8_08 Datasheet, PDF (32/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
13. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between
XTAL1 and GND, and XTAL2 and GND.
Figure 13. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
Timer/Counter
Oscillator
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 12.
Table 12. Start-up Times for the External Clock Selection
SUT1..0
00
Start-up Time from
Power-down and
Power-save
6 CK
Additional Delay
from Reset
(V_CC = 5.0V)
–
Recommended Usage
BOD enabled
01
6 CK
4.1 ms
Fast rising power
10
6 CK
65 ms
Slowly rising power
11
Reserved
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from
one clock cycle to the next can lead to unpredictable behavior. It is required to ensure that the
MCU is kept in Reset during such changes in the clock frequency.
For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is
connected directly between the pins. By programming the CKOPT Fuse, the user can enable
internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors.
The Oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external clock
source to TOSC1 is not recommended.
Note: The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency Oscillator
and the internal capacitors have the same nominal value of 36 pF.
32 ATmega8(L)
2486T–AVR–05/08