AT90PWM3 datasheet(31/361 Pages) ATMEL

31 / 361 page

4317J–AVR–08/10

AT90PWM2/3/2B/3B

up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts

from reset, there is an additional delay allowing the power to reach a stable level before starting

normal operation. The Watchdog Oscillator is used for timing this real-time part of the start-up

time. The number of WDT Oscillator cycles used for each time-out is shown in Table 7-3. The

frequency of the Watchdog Oscillator is voltage dependent as shown in “Watchdog Oscillator

Frequency vs. VCC” on page 328.

7.3

Default Clock Source

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default

clock source setting is the Internal RC Oscillator with longest start-up time and an initial system

clock prescaling of 8. This default setting ensures that all users can make their desired clock

source setting using an In-System or Parallel programmer.

7.4

Low Power 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 7-3. Either a quartz crystal or a

ceramic resonator may be used.

This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 out-

put. It gives the lowest power consumption, but is not capable of driving other clock inputs.

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 7-4. For ceramic resonators, the capacitor values given by

the manufacturer should be used. For more information on how to choose capacitors and other

details on Oscillator operation, refer to the Multi-purpose Oscillator Application Note.

Figure 7-3.

Crystal Oscillator Connections

Table 7-3.

Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V)

Typ Time-out (VCC = 3.0V)

Number of Cycles

4.1 ms

4.3 ms

4K (4,096)

65 ms

69 ms

64K (65,536)

XTAL2

XTAL1

GND

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