33
8025I–AVR–02/09
ATmega48P/88P/168P/328P
6.6 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 Table
26-1 on page 319 for more details. The device is shipped with the CKDIV8 Fuse programmed.
See ”System Clock Prescaler” on page 35 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 6-10. 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
Table 26-1 on page 319.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 37, 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 Table 26-1 on page 319.
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 299.
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 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
Table 6-11 on page 33.
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.
6.7 128 kHz Internal Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3V and 25°C. This clock may be select as the system clock by
programming the CKSEL Fuses to “11” as shown in Table 6-12.
Table 6-10. Internal Calibrated RC Oscillator Operating Modes
(2)
Frequency Range
(1)
(MHz) CKSEL3..0
7.3 - 8.1 0010
Table 6-11. 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
(1)
00
Fast rising power 6 CK 14CK + 4.1 ms 01
Slowly rising power 6 CK 14CK + 65 ms
(2)
10
Reserved 11
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