Datasheet

41
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum V
CC
. The delay will not mon-
itor 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 considered 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.
10.4 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 10-2. 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 output. It gives the low-
est 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 42.
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 environ-
ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 10-3 on page 42. For
ceramic resonators, the capacitor values given by the manufacturer should be used.
Figure 10-2. Crystal Oscillator Connections
The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range. The
operating mode is selected by the fuses CKSEL3:1 as shown in Table 10-3 on page 42.
Table 10-2. Number of Watchdog Oscillator Cycles
Typical Time-out (V
CC
= 5.0V) Typical Time-out (V
CC
= 3.0V) Number of Cycles
0ms 0ms 0
4.1ms 4.3ms 512
65ms 69ms 8K (8,192)
XTAL2
XTAL1
GND
C2
C1