ATMEGA48 datasheet(33/374 Pages) ATMEL

33 / 374 page

2545J–AVR–12/06

ATmega48/88/168

7.5

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 7-2. When this

Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in

Table 7-7.

Note:

1. These options should only be used if frequency stability at start-up is not important for the

application.

7.6

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 36 for

more details.

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in

Table 7-8. 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 27-1 on page 306.

By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on

page 37, 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 27-1 on page 306.

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 288.

Notes:

1. The device is shipped with this option selected.

2. The frequency ranges are preliminary values.

Table 7-7.

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

(VCC = 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 7-8.

Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

ATMEGA48 Datasheet(PDF) 33 Page - ATMEL Corporation

ATMEGA48 Datasheet(HTML) 33 Page - ATMEL Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL