33
7799D–AVR–11/10
ATmega8U2/16U2/32U2
Notes: 1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These
options are not suitable for crystals.
They can also be used with crystals when not operating close to the maximum frequency of the device, and
if frequency stability at start-up is not important for the application.
8.6
Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the the user. See
Table26-1 on page 266 for more details. The device is shipped with the CKDIV8 Fuse programmed.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-6. 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 cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
page 38, it is possible to get a higher calibration accuracy than by using the factory calibration.
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 cali-
Table 1.
Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC = 5.0V)
CKSEL0
SUT1..0
Ceramic resonator, fast
rising power
258 CK
000
Ceramic resonator,
slowly rising power
258 CK
001
Ceramic resonator,
BOD enabled
1K CK
010
Ceramic resonator, fast
rising power
1K CK
011
Ceramic resonator,
slowly rising power
1K CK
100
Crystal Oscillator, BOD
enabled
16K CK
14CK
1
01
Crystal Oscillator, fast
rising power
16K CK
14CK + 4.1 ms
1
10
Crystal Oscillator,
slowly rising power
16K CK
14CK + 65 ms
1
11
Table 8-6.
Internal Calibrated RC Oscillator Operating Modes
(3)CKSEL3..0
7.3 - 8.1