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ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
9.3
Default clock source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source set-
ting is the Internal RC Oscillator with longest start-up time and an initial system clock prescaling of 8. This default
setting ensures that all users can make their desired clock source setting using an In-System or Parallel
programmer.
9.4
Calibrated internal RC oscillator
By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage and temperature
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 9-3. If
selected, it will operate with no external components. During reset, hardware loads the pre-programmed calibration
value into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The accuracy of this cali-
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer
and for the Reset Time-out. For more information on the pre-programmed calibration value, see the section
”Cali-Notes:
1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values.
3. If 8MHz frequency exceeds the specification of the device (depends on V
CC), the CKDIV8 Fuse can be pro-
grammed in order to divide the internal frequency by 8.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 9-4.
Note:
1. The device is shipped with this option selected
.
Table 9-3.
Internal calibrated RC oscillator operating modes
CKSEL3:0
7.3 - 8.1
0010
Table 9-4.
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
6CK
14CK
00
Fast rising power
6CK
14CK + 4.1ms
01
Slowly rising power
6CK
10
Reserved
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