ATmega48/88/168
6.5
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 6-2. When this
Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 6-7.
Table 6-7.
Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
Power Conditions
BOD enabled
(VCC = 5.0V)
CKSEL0
SUT1..0
00
1K CK
1K CK
14CK(1)
0
0
0
0
1
1
1
1
Fast rising power
Slowly rising power
14CK + 4.1 ms(1)
14CK + 65 ms(1)
01
1K CK
10
Reserved
32K CK
32K CK
32K CK
Reserved
11
BOD enabled
14CK
00
Fast rising power
Slowly rising power
14CK + 4.1 ms
14CK + 65 ms
01
10
11
Note:
1. These options should only be used if frequency stability at start-up is not important for the
application.
6.6
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 6-8. 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 10ꢀ. The oscillator can be calibrated to any frequency in the range
7.3 - 8.1 MHz within 2ꢀ 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.
Table 6-8.
Internal Calibrated RC Oscillator Operating Modes(1)(3)
Frequency Range(2) (MHz)
CKSEL3..0
0010
7.3 - 8.1
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 VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
31
2545E–AVR–02/05
MICROCHIP [ MICROCHIP ]
