ATmega644PA datasheet(34/468 Pages) ATMEL

34 / 468 page

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

Notes:

1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

8.4

Full Swing Crystal Oscillator

This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is

useful for driving other clock inputs and in noisy environments. The current consumption is

higher than the ”Low Power Crystal Oscillator” on page 33. Note that the Full Swing Crystal

Oscillator will only operate for Vcc = 2.7 - 5.5 volts.

Some initial guidelines for choosing capacitors for use with crystals are given in Table 8-6. The

crystal should be connected as described in ”Clock Source Connections” on page 32.

The operating mode is selected by the fuses CKSEL3..1 as shown in Table 8-5.

Notes:

1. 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. It must be ensured

that the resulting divided clock meets the frequency specification of the device.

Crystal Oscillator, BOD

enabled

16K CK

14CK

Crystal Oscillator, fast

rising power

16K CK

14CK + 4.1 ms

Crystal Oscillator, slowly

rising power

16K CK

14CK + 65 ms

Table 8-4.

Start-up Times for the Low Power Crystal Oscillator Clock Selection (Continued)

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

CKSEL0

SUT1..0

Table 8-5.

Full Swing Crystal Oscillator Operating Modes

Frequency Range(1) (MHz)

CKSEL3..1

Recommended Range for Capacitors C1

and C2 (pF)

0.4 - 20

011

12 - 22

Table 8-6.

Start-up Times for the Full Swing Crystal Oscillator Clock Selection

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

CKSEL0

SUT1..0

Ceramic resonator, fast

rising power

258 CK

14CK + 4.1 ms(1)

000

Ceramic resonator, slowly

rising power

258 CK

14CK + 65 ms(1)

001

Ceramic resonator, BOD

enabled

1K CK

14CK(2)

010

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