ATMEGA640 datasheet(45/449 Pages) ATMEL

45 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Note:

1. The device is shipped with this option selected.

128 kHz Internal

Oscillator

The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz.

The frequency is nominal at 3V and 25

°C. This clock may be select as the system clock

by programming the CKSEL Fuses to “11” as shown in Table 19.

Note:

1. The frequency is preliminary value. Actual value is TBD.

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

shown in Table 20.

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in

Figure 23. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to “0000”.

Figure 23. External Clock Drive Configuration

Table 18. Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

CC = 5.0V)

SUT1:0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms(1)

Reserved

Table 19. 128 kHz Internal Oscillator Operating Modes

Nominal Frequency

CKSEL3:0

128 kHz

0011

Table 20. Start-up Times for the 128 kHz Internal Oscillator

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset

SUT1:0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4 ms

Slowly rising power

6 CK

14CK + 64 ms

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

ATMEGA640 Datasheet(PDF) 45 Page - ATMEL Corporation

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