ATMEGA48-15AZV 데이터 시트보기 (PDF) - Atmel Corporation
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ATMEGA48-15AZV Datasheet PDF : 340 Pages
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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
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
Start-up Time from
Power-down and
Power-save
1K CK
1K CK
1K CK
Additional Delay
from Reset
(VCC = 5.0V)
14CK(1)
14CK + 4.1 ms(1)
14CK + 65 ms(1)
CKSEL0
0
0
0
SUT1..0
00
01
10
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
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 33 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 ± 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
more information on the pre-programmed calibration value, see the section “Calibration Byte” on
page 282.
Table 6-8.
Internal Calibrated RC Oscillator Operating Modes(1)(3)
Frequency Range(2) (MHz)
7.3 - 8.1
CKSEL3..0
0010
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.
30 Atmel ATmega48/88/168 Automotive
7530J–AVR–03/12
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