ATMEGA162 datasheet(36/325 Pages) ATMEL

36 / 325 page

ATmega162/V

2513I–AVR–02/07

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as

shown below. The clock from the selected source is input to the AVR clock generator,

and routed to the appropriate modules.

Note:

For all fuses “1” means unprogrammed while “0” means programmed.

The various choices for each clocking option is given in the following sections. When the

CPU wakes up from Power-down or Power-save, the selected clock source is used to

time the start-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 commencing normal operation. The Watchdog Oscillator is

used for timing this realtime part of the start-up time. The number of WDT Oscillator

cycles used for each Time-out is shown in Table 6. The frequency of the Watchdog

Oscillator is voltage dependent as shown in “ATmega162 Typical Characteristics” on

page 277.

Default Clock Source

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

default clock source setting is therefore the Internal RC Oscillator with longest startup

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.

Crystal Oscillator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can

be configured for use as an On-chip Oscillator, as shown in Figure 19. Either a quartz

crystal or a ceramic resonator may be used.

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 capac-

itance, and the electromagnetic noise of the environment. Some initial guidelines for

choosing capacitors for use with crystals are given in Table 7. For ceramic resonators,

the capacitor values given by the manufacturer should be used.

Table 5. Device Clocking Options Select

Device Clocking Option

CKSEL3..0

External Crystal/Ceramic Resonator

1111 - 1000

External Low-frequency Crystal

0111 - 0100

Calibrated Internal RC Oscillator

0010

External Clock

0000

Reserved

0011, 0001

Table 6. Number of Watchdog Oscillator Cycles

Typ Time-out (V

CC = 5.0V)

Typ Time-out (V

CC = 3.0V)

Number of Cycles

4.1 ms

4.3 ms

4K (4,096)

65 ms

69 ms

64K (65,536)

ATMEGA162 Datasheet(PDF) 36 Page - ATMEL Corporation

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