ATtiny261A datasheet(26/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

mencing normal operation. The watchdog oscillator is used for timing this real-time part of the

start-up time. The number of WD oscillator cycles used for each time-out is shown in Table 6-2.

6.2.1

External Clock

To drive the device from an external clock source, CLKI should be driven as shown in Figure 6-

2. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

Figure 6-2.

External Clock Drive Configuration

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

Table 6-3.

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from

one clock cycle to the next can lead to unpredictable behavior. It is required to ensure that the

MCU is kept in Reset during such changes in the clock frequency.

Note that the system clock prescaler can be used to implement run-time changes of the internal

clock frequency. See “System Clock Prescaler” on page 31 for details.

6.2.2

High-Frequency PLL Clock

The internal PLL generates a clock signal with a frequency eight times higher than the source

input. The PLL uses the output of the internal 8 MHz oscillator as source and the default setting

generates a fast peripheral clock signal of 64 MHz.

Table 6-2.

Number of Watchdog Oscillator Cycles

Typ Time-out

Number of Cycles

4 ms

512

64 ms

8K (8,192)

Table 6-3.

Start-up Times for the External Clock Selection

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

EXTERNAL

CLOCK

SIGNAL

CLKI

GND

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