34
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
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 8.0MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See
for more details. The device is shipped with the CKDIV8 Fuse programmed.
”System Clock Prescaler” on page 36
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
. 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 cali-
brates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
.
By changing the OSCCAL register from SW, see
”OSCCAL – Oscillator Calibration Register” on
, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in
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-
bration value, see the section
”Calibration Byte” on page 302
Notes:
1. The device is shipped with this option selected.
2. If 8MHz frequency exceeds the specification of the device (depends on V
CC
), 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 + 4.1ms 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
CKSEL3...
0
Start-up Time from
Power-down and Power-save
Recommended Usage
0100
1K CK
0101
32K CK
Stable frequency at start-up
Table 9-11.
Internal Calibrated RC Oscillator Operating Modes
Frequency Range
(MHz)
CKSEL3...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)
SUT1...0
BOD enabled
6 CK
14CK
00
Fast rising power
6 CK
14CK + 4.1ms
01
Slowly rising power
6 CK
14CK + 65ms
10
Reserved
11