ATtiny261A datasheet(29/292 Pages) ATMEL

29 / 292 page

8197A–AVR–10/09

ATtiny261A/461A/861A

It is possible to reach a higher accuracy than factory calibration by changing the OSCCAL regis-

ter from software. See “OSCCAL – Oscillator Calibration Register” on page 32. The accuracy of

this calibration is shown as User calibration in Table 19-2 on page 186.

When this oscillator is used as device 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 cali-

bration value, see section “Calibration Byte” on page 169.

6.2.4

Internal 128 kHz Oscillator

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

quency depends on supply voltage, temperature and batch variations. 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 6-8.

Note:

1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to

ensure programming mode can be entered.

6.2.5

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 6-4. To find suitable capacitors please consult the manufacturer’s datasheet.

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

Table 6-9.

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

Table 6-8.

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(1)

BOD enabled

6 CK

14CK + 4 ms

Fast rising power

6 CK

14CK + 64 ms

Slowly rising power

Reserved

Table 6-9.

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

SUT1:0

Start-up Time

from Power Down

Additional Delay

from Reset

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

Table 6-10.

Capacitance for Low-Frequency Crystal Oscillator

Device

32 kHz Osc. Type

Cap (Xtal1/Tosc1)

Cap (Xtal2/Tosc2)

ATtiny261A/461A/861A

System Osc.

16 pF

6 pF

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