Rainbow Electronics ATmega162V - Manual (Page 34)

34
ATmega162/V
2513E–AVR–09/03
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 274.
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)