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ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
30
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
Table 29-3 on page 271 for more details. The device is shipped
This clock may be selected as the system clock by programming the CKSEL fuses as shown in
Table 9-1 on page 25. If
selected, it will operate with no external components. During reset, hardware loads the pre-programmed default 3V
calibration value into the OSCCAL register and thereby automatically calibrates the RC oscillator for 3V operation. If the
device is to be used at 5V then the alternate RC oscillator 5V calibration Byte (
Table 27-5 on page 244) can be read from
signature row and stored into the OSCCAL register by the user application program for better 5V frequency accuracy. The
possible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is shown
When this oscillator is used as the chip clock, the watchdog oscillator will still be used for the watchdog timer and for the
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 9-12.9.7
128kHz Internal Oscillator
The 128kHz internal oscillator is a low power oscillator providing a clock of 128kHz. The frequency 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 9-13.Table 9-11. Internal Calibrated RC Oscillator Operating Modes
Frequency Range(2) (MHz)
CKSEL3...0
7.3 - 8.1
0010(1)
Notes: 1.
The device is shipped with this option selected.
2.
If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be pro-
grammed in order to divide the internal frequency by 8.
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
(VCC = 5.0V)
SUT1...0
BOD enabled
6CK
14CK(1)
00
Fast rising power
6CK
14CK + 4.1ms
01
Slowly rising power
6CK
14CK + 65ms(2)
10
Reserved
11
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 9-13. 128kHz Internal Oscillator Operating Modes
Nominal Frequency(1)
CKSEL3...0
128kHz
0011
Note:
1.
Note that the 128kHz oscillator is a very low power clock source, and is not designed for high accuracy.