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2513L–AVR–03/2013
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-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)