Datasheet

31
2545F–AVR–06/05
ATmega48/88/168
7.5 Low Frequency Crystal Oscillator
The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre-
quency Crystal Oscillator. The crystal should be connected as shown in Figure 7-2. When this
Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 7-7.
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
7.6 Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nom-
inal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
”System Clock Prescaler” on page 34 for more details. This clock may be selected as the system
clock by programming the CKSEL Fuses as shown in Table 7-8. If selected, it will operate with
no external components. During reset, hardware loads the calibration byte into the OSCCAL
Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this calibration
gives a frequency of 8 MHz ±10%. The oscillator can be calibrated to any frequency in the range
7.3 - 8.1 MHz within ± 2% accuracy, by changing the OSCCAL register. 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 calibration value, see the sec-
tion ”Calibration Byte” on page 284.
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. 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.
Table 7-7. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) CKSEL0 SUT1..0
BOD enabled 1K CK 14CK
(1)
000
Fast rising power 1K CK 14CK + 4.1 ms
(1)
001
Slowly rising power 1K CK 14CK + 65 ms
(1)
010
Reserved 0 11
BOD enabled 32K CK 14CK 1 00
Fast rising power 32K CK 14CK + 4.1 ms 1 01
Slowly rising power 32K CK 14CK + 65 ms 1 10
Reserved 1 11
Table 7-8. Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz) CKSEL3..0
7.3 - 8.1 0010