Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 40
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum Vcc. The delay will not
monitor the actual voltage and it will be required to select a delay longer than the Vcc rise time. If this is not
possible, an internal or external Brown-Out Detection circuit should be used. A BOD circuit will ensure sufficient
Vcc 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, Vcc is assumed to be at a
sufficient level and only the start-up time is included.
9.2.3 Clock source connections
The 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 9-2 on page 40. 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. For ceramic resonators, the capacitor values given by the manufacturer should be used.
Figure 9-2. Crystal oscillator connections
Table 9-2. 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