Data Sheet

29
2545K–AVR–04/07
ATmega48/88/168
selectable delays are shown in Table 7-2. The frequency of the Watchdog Oscillator is voltage
dependent as shown in ”Typical Characteristics – Preliminary Data” on page 315.
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum V
CC
. The
delay will not monitor the actual voltage and it will be required to select a delay longer than the
V
CC
rise time. If this is not possible, an internal or external Brown-Out Detection circuit should be
used. A BOD circuit will ensure sufficient V
CC
before it releases the reset, and the time-out delay
can be disabled. Disabling the time-out delay without utilizing a Brown-Out Detection circuit is
not recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is consid-
ered stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal
reset active for a given number of clock cycles. The reset is then released and the device will
start to execute. The recommended oscillator start-up time is dependent on the clock type, and
varies from 6 cycles for an externally applied clock to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up time when
the device starts up from reset. When starting up from Power-save or Power-down mode, V
CC
is
assumed to be at a sufficient level and only the start-up time is included.
7.3 Low Power Crystal Oscillator
Pins 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 7-2. Either a quartz crystal or a
ceramic resonator may be used.
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 out-
put. It gives the lowest power consumption, but is not capable of driving other clock inputs, and
may be more susceptible to noise in noisy environments. In these cases, refer to the ”Full Swing
Crystal Oscillator” on page 31.
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 7-3. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Table 7-2. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V) Typ Time-out (V
CC
= 3.0V) Number of Cycles
0 ms 0 ms 0
4.1 ms 4.3 ms 4K (4,096)
65 ms 69 ms 8K (8,192)