ATmega48 datasheet(30/340 Pages) ATMEL

30 / 340 page

7530H–AVR–02/09

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

Table 6-7.

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

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.

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

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

BOD enabled

32K CK

14CK

Fast rising power

32K CK

14CK + 4.1 ms

Slowly rising power

32K CK

14CK + 65 ms

Reserved

Table 6-8.

Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

ATmega48 Datasheet(PDF) 30 Page - ATMEL Corporation

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