
34
8025M–AVR–6/11
ATmega48P/88P/168P
Note:
1. This option should only be used if frequency stability at start-up is not important for the
application
9.6
Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See
Table29-1 on page 313 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 9-11. 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-
Notes:
1. This is the recommended CKSEL settings for the different frequency ranges.
2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed 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
Note:
1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1 ms to ensure programming mode can be entered.
2.
The device is shipped with this option selected.
Table 9-10.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL[3:0]
Start-up Time from
Power-down and Power-save
Recommended Usage
1K CK
0101
32K CK
Stable frequency at start-up
Table 9-11.
Internal Calibrated RC Oscillator Operating Modes
CKSEL[3:0]
7.3 - 8.1
0010
Table 9-12.
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)
SUT[1:0]
BOD enabled
6 CK
00
Fast rising power
6 CK
14CK + 4.1 ms
01
Slowly rising power
6 CK
10
Reserved
11