ATmega162 datasheet(36/324 Pages) ATMEL

36 / 324 page

2513K–AVR–07/09

ATmega162/V

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 com-

mencing 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 Typi-

cal Characteristics” on page 275.

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 con-

figured 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 capacitance, 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)

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