ATmega162 datasheet(40/324 Pages) ATMEL

40 / 324 page

2513K–AVR–07/09

ATmega162/V

External Clock

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

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

Figure 20. External Clock Drive Configuration

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

Table 14.

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 while still ensuring stable operation. Refer to “System Clock Prescaler” on page

41 for details.

Clock output

buffer

When the CKOUT Fuse is programmed, the system clock will be output on PortB 0. 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 PortB will be overridden when the fuse is pro-

Table 13. Internal RC Oscillator Frequency Range.

OSCCAL Value

Min Frequency in Percentage of

Nominal Frequency

Max Frequency in Percentage of

Nominal Frequency

0x00

50%

100%

0x3F

75%

150%

0x7F

100%

200%

Table 14. 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

–

BOD enabled

6 CK

4.1 ms

Fast rising power

6 CK

65 ms

Slowly rising power

Reserved

EXTERNAL

CLOCK

SIGNAL

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