Datasheet

34
8023F–AVR–07/09
ATmega325P/3250P
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-9 on page 34.
Note: 1. The device is shipped with this option selected.
8.7 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 3. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 2.
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
Table 8-9. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) SUT1..0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1 ms 01
Slowly rising power 6 CK 14CK + 65 ms
(1)
10
Reserved 11
Table 1. Crystal Oscillator Clock Frequency
CKSEL3..0 Frequency Range
0000 0 - 16 MHz
Table 2. 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 14CK BOD enabled
01 6 CK 14CK + 4.1 ms Fast rising power
10 6 CK 14CK + 65 ms Slowly rising power
11 Reserved
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND