ATmega16U4 datasheet(30/408 Pages) ATMEL

30 / 408 page

7766D–AVR–01/09

ATmega16U4/ATmega32U4

electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for

use with crystals are given in Table 6-3. For ceramic resonators, the capacitor values given by

the manufacturer should be used.

Figure 6-2.

Crystal Oscillator Connections

The Low Power Oscillator can operate in three different modes, each optimized for a specific fre-

quency range. The operating mode is selected by the fuses CKSEL[3..1] as shown in Table 6-3.

Notes:

1. The frequency ranges are preliminary values. Actual values are TBD.

2. This option should not be used with crystals, only with ceramic resonators.

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. It must be ensured

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

The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table

6-4.

Table 6-3.

Low Power Crystal Oscillator Operating Modes(3)

Frequency Range(1) (MHz)

CKSEL3..1

Recommended Range for Capacitors

C1 and C2 (pF)

0.4 - 0.9

100(2)

–

0.9 - 3.0

101

12 - 22

3.0 - 8.0

110

12 - 22

8.0 - 16.0

111

12 - 22

Table 6-4.

Start-up Times for the Low Power 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

Ceramic resonator, fast

rising power

1K CK

14CK + 4.1 ms(2)

011

Ceramic resonator,

slowly rising power

1K CK

14CK + 65 ms(2)

100

XTAL2

XTAL1

GND

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