ATtiny261-15MZ datasheet(30/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

7.7

128 kHz Internal Oscillator

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

quency is nominal at 3V and 25°C. This clock may be select as the system clock by

programming the CKSEL Fuses to “0011”.

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

Table 7-8.

7.8

Low-frequency Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crystal

oscillator must be selected by setting CKSEL fuses to ‘0100’. The crystal should be connected

as shown in Figure 7-4. Refer to the 32 kHz Crystal Oscillator Application Note for details on

oscillator operation and how to choose appropriate values for C1 and C2.

When this oscillator is selected, start-up times are determined by the SUT fuses as shown in

Table 7-9.

Notes:

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

application.

7.9

Crystal Oscillator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-

figured for use as an On-chip Oscillator, as shown in Figure 7-4. Either a quartz crystal or a

ceramic resonator may be used.

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the

capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the

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

use with crystals are given in Table 7-10. For ceramic resonators, the capacitor values given by

the manufacturer should be used.

Table 7-8.

Start-up Times for the 128 kHz Internal Oscillator

SUT1..0

Start-up Time from

Power-down and Power-save

Additional Delay from

Reset

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4 ms

Fast rising power

6 CK

14CK + 64 ms

Slowly rising power

Reserved

Table 7-9.

Start-up Times for the Low Frequency Crystal Oscillator Clock Selection

SUT1..0

Start-up Time from Power

Down and Power Save

Additional Delay from

Reset (VCC = 5.0V)

Recommended usage

1K (1024) CK(1)

4 ms

Fast rising power or BOD enabled

1K (1024) CK(1)

64 ms

Slowly rising power

32K (32768) CK

64 ms

Stable frequency at start-up

Reserved

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