ATMEGA640 datasheet(44/449 Pages) ATMEL

44 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Low Frequency Crystal

Oscillator

The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low

Frequency Crystal Oscillator. The crystal should be connected as shown in Figure 21.

When this Oscillator is selected, start-up times are determined by the SUT Fuses and

CKSEL0 as shown in Table 16.

Note:

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

for the application.

Calibrated Internal RC

Oscillator

By defaylt, the Internal RC Oscillator provides an approximate 8 MHz clock. Though volt-

age and temperature dependent, this clock can be very accurately calibrated by the

user. See Table 172 on page 384 and “Internal Oscillator Speed” on page 409 for more

details. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock

Prescaler” on page 47 for more details.

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

shown in Table 17. 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 calibrates the RC Oscillator. The accuracy of this calibration is

shown as Factory calibration in Table 172 on page 384.

By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Reg-

ister” on page 48, 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

172 on page 384.

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 345.

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.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as

shown in Table 18 on page 45.

Table 16. 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 17. Internal Calibrated RC Oscillator Operating Modes

(1)(3)

Frequency Range(2) (MHz)

CKSEL3:0

7.3 - 8.1

0010

ATMEGA640 Datasheet(PDF) 44 Page - ATMEL Corporation

ATMEGA640 Datasheet(HTML) 44 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