ATmega128 datasheet(119/386 Pages) ATMEL

119 / 386 page

119

2467S–AVR–07/09

ATmega128

are generated on the output compare outputs OCnx. For more details about advanced counting

sequences and waveform generation, see “Modes of Operation” on page 124.

The Timer/Counter Overflow (TOVn) flag is set according to the mode of operation selected by

the WGMn3:0 bits. TOVn can be used for generating a CPU interrupt.

Input Capture Unit

The Timer/Counter incorporates an Input Capture unit that can capture external events and give

them a time-stamp indicating time of occurrence. The external signal indicating an event, or mul-

tiple events, can be applied via the ICPn pin or alternatively, for the Timer/Counter1 only, via the

Analog Comparator unit. The time-stamps can then be used to calculate frequency, duty-cycle,

and other features of the signal applied. Alternatively the time-stamps can be used for creating a

log of the events.

The Input Capture unit is illustrated by the block diagram shown in Figure 48. The elements of

the block diagram that are not directly a part of the Input Capture unit are gray shaded. The

small “n” in register and bit names indicates the Timer/Counter number.

Figure 48. Input Capture Unit Block Diagram

Note:

The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not

Timer/Counter3.

When a change of the logic level (an event) occurs on the Input Capture Pin (ICPn), alternatively

on the analog Comparator output (ACO), and this change confirms to the setting of the edge

detector, a capture will be triggered. When a capture is triggered, the 16-bit value of the counter

(TCNTn) is written to the Input Capture Register (ICRn). The Input Capture Flag (ICFn) is set at

the same system clock as the TCNTn value is copied into ICRn Register. If enabled (TICIEn =

1), the Input Capture flag generates an Input Capture interrupt. The ICFn flag is automatically

cleared when the interrupt is executed. Alternatively the ICFn flag can be cleared by software by

writing a logical one to its I/O bit location.

Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the low

byte (ICRnL) and then the high byte (ICRnH). When the low byte is read the high byte is copied

ICFn (Int.Req.)

Analog

Comparator

WRITE

ICRn (16-bit Register)

ICRnH (8-bit)

Noise

Canceler

ICPn

Edge

Detector

TEMP (8-bit)

DATA BUS (8-bit)

ICRnL (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit)

TCNTnL (8-bit)

ACIC*

ICNC

ICES

ACO*

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