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ATMEGA1280 데이터시트(HTML) 41 Page - ATMEL Corporation

부품명 ATMEGA1280
상세내용  8- BIT Microcontroller with 256K Bytes In-System Programmable Flash
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제조사  ATMEL [ATMEL Corporation]
홈페이지  http://www.atmel.com
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ATMEGA1280 데이터시트(HTML) 41 Page - ATMEL Corporation

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ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
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.
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 22. 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 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 43.
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 9. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
Figure 22. 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 9.
XTAL2
XTAL1
GND
C2
C1


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