ATmega128 datasheet(93/386 Pages) ATMEL

93 / 386 page

2467S–AVR–07/09

ATmega128

8-bit

Timer/Counter0

with PWM and

Asynchronous

Operation

Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The main

features are:

• Single Channel Counter

• Clear Timer on Compare Match (Auto Reload)

• Glitch-free, Phase Correct Pulse Width Modulator (PWM)

• Frequency Generator

• 10-bit Clock Prescaler

• Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)

• Allows Clocking from External 32 kHz Watch Crystal Independent of the I/O Clock

Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 34. For the actual place-

ment of I/O pins, refer to “Pin Configurations” on page 2. CPU accessible I/O registers, including

I/O bits and I/O pins, are shown in bold. The device-specific I/O register and bit locations are

listed in the “8-bit Timer/Counter Register Description” on page 104.

Figure 34. 8-bit Timer/Counter Block Diagram

Registers

The Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers. Interrupt

request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR).

All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and

TIMSK are not shown in the figure since these registers are shared by other timer units.

The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from

the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by

the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock

source the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-

Timer/Counter

TCNTn

Waveform

Generation

OCn

= 0

Control Logic

= 0xFF

TOP

BOTTOM

count

clear

direction

TOVn

(Int.Req.)

OCn

(Int.Req.)

Synchronization Unit

OCRn

TCCRn

ASSRn

Status flags

clk

I/O

clk

ASY

Synchronized Status flags

asynchronous mode

select (ASn)

TOSC1

T/C

Oscillator

TOSC2

Prescaler

clk

I/O

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