AT90PWM2 datasheet(36/365 Pages) ATMEL

36 / 365 page

AT90PWM2/3

4317B–AVR–02/05

128 kHz Internal

Oscillator

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

The frequency is nominal at 3V and 25

°C. This clock is used by the Watchdog

Oscillator.

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in

Figure 16. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to “0000”.

Figure 16. External Clock Drive Configuration

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

shown in Table 10.

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

clock frequency 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 while still ensuring stable operation. Refer to “System Clock

Prescaler” on page 37 for details.

Clock Output Buffer

When the CKOUT Fuse is programmed, the system Clock will be output on CLKO. This

mode is suitable when chip clock is used to drive other circuits on the system. The clock

will be output also during reset and the normal operation of I/O pin will be overridden

when the fuse is programmed. Any clock source, including internal RC Oscillator, can be

selected when CLKO serves as clock output. If the System Clock Prescaler is used, it is

the divided system clock that is output (CKOUT Fuse programmed).

Table 9. External Clock Frequency

CKSEL3..0

Frequency Range

0000

0 - 16 MHz

Table 10. Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (VCC = 5.0V)

Recommended Usage

6 CK

14CK

BOD enabled

6 CK

14CK + 4.1 ms

Fast rising power

6 CK

14CK + 65 ms

Slowly rising power

Reserved

XTAL2

XTAL1

GND

External

Clock

Signal

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