8.5
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 8-2. When this
Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in
Table 8-7.
Table 8-7.
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.
8.6
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. The device
is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 37 for
more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-8. 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 28-1 on page 307.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 38, 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 28-1 on page 307.
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 289.
34
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
