30
ATmega8A [DATASHEET]
8159E–AVR–02/2013
9.7
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 9-4. 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 9-4.
External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 9-10.When applying an external clock, it is required to avoid sudden changes in the applied clock frequency 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.
9.8
Timer/Counter Oscillator
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.
Table 9-10.
Start-up Times for the External Clock Selection
SUT1:0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC = 5.0V)
Recommended Usage
00
6 CK
–
BOD enabled
01
6 CK
4.1 ms
Fast rising power
10
6 CK
65 ms
Slowly rising power
11
Reserved
EXTERNAL
CLOCK
SIGNAL