30
7530J–AVR–03/12
Atmel ATmega48/88/168 Automotive
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
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
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 ± 1%. The tolerance of the internal RC oscillator remains better than
±10% within the whole automotive temperature and voltage ranges (2.7V to 5.5V, -40°C to
+125°C). The oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1%
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
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 6-7.
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
000
Fast rising power
1K CK
001
Slowly rising power
1K CK
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 6-8.
Internal Calibrated RC Oscillator Operating
ModesCKSEL3..0
7.3 - 8.1
0010