ATMEGA8L ATMEL [ATMEL Corporation], ATMEGA8L Datasheet - Page 30

• Manufacturer Part Number: ATMEGA8L
• Description: 8-bit AVR with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

External Clock
Timer/Counter Oscillator
30
ATmega8(L)
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 pro-
grammed to “0000”. By programming the CKOPT Fuse, the user can enable an internal
36 pF capacitor between XTAL1 and GND.
Figure 13. External Clock Drive Configuration
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
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.
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 crys-
tal. Applying an external clock source to TOSC1 is not recommended.
SUT1..0
00
01
10
11
Start-up Time from
Power-down and
Power-save
6 CK
6 CK
6 CK
EXTERNAL
SIGNAL
CLOCK
Additional Delay
(V
from Reset
CC
4.1 ms
Reserved
65 ms
= 5.0V)
–
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
2486M–AVR–12/03