ATMEGA640 datasheet(165/436 Pages) MICROCHIP

Part #	ATMEGA640

Description	8-bit Microcontroller with 16/32/64KB In-System Programmable Flash

PDF	436 Pages

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Manufacturer	MICROCHIP [Microchip Technology]

Direct Link	http://www.microchip.com

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Each half period of the external clock applied must be longer than one system clock cycle to ensure correct sam-

pling. The external clock must be guaranteed to have less than half the system clock frequency (fExtClk < fclk_I/O/2)

given a 50/50% duty cycle. Since the edge detector uses sampling, the maximum frequency of an external clock it

can detect is half the sampling frequency (Nyquist sampling theorem). However, due to variation of the system

clock frequency and duty cycle caused by Oscillator source (crystal, resonator, and capacitors) tolerances, it is rec-

ommended that maximum frequency of an external clock source is less than fclk_I/O/2.5.

An external clock source can not be prescaled.

Figure 18-2. Prescaler for synchronous Timer/Counters

PSR10

Clear

clk

I/O

Synchronization

TIMER/COUNTERn CLOCK SOURCE

clk

TIMER/COUNTERn CLOCK SOURCE

clk

CSn0

CSn1

CSn2

CSn0

CSn1

CSn2

ATMEGA640 Datasheet(PDF) 165 Page - Microchip Technology