ATmega644PA datasheet(239/468 Pages) ATMEL

239 / 468 page

239

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

21. AC - Analog Comparator

21.1

Overview

The Analog Comparator compares the input values on the positive pin AIN0 and negative pin

AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin

AIN1, the Analog Comparator output, ACO, is set. The comparator’s output can be set to trigger

the Timer/Counter1 Input Capture function. In addition, the comparator can trigger a separate

interrupt, exclusive to the Analog Comparator. The user can select Interrupt triggering on com-

parator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is

shown in Figure 21-1.

The Power Reduction ADC bit, PRADC, in ”PRR – Power Reduction Register” on page 49 must

be disabled by writing a logical zero to be able to use the ADC input MUX.

Figure 21-1. Analog Comparator Block Diagram

(2)

Notes:

1. See Table 21-1 on page 239.

2. Refer to Figure 1-1 on page 2 and Table 13-5 on page 83 for Analog Comparator pin

placement.

21.2

Analog Comparator Multiplexed Input

It is possible to select any of the ADC7..0 pins to replace the negative input to the Analog Com-

parator. The ADC multiplexer is used to select this input, and consequently, the ADC must be

switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in

ADCSRB) is set and the ADC is switched off (ADEN in ADCSRA is zero), MUX2..0 in ADMUX

select the input pin to replace the negative input to the Analog Comparator, as shown in Table

21-1 on page 239. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the

Analog Comparator.

ACBG

BANDGAP

REFERENCE

ADC MULTIPLEXER

OUTPUT

ACME

ADEN

(1)

Table 21-1.

Analog Comparator Mulitiplexed Input

ACME

ADEN

MUX2..0

Analog Comparator Negative Input

xxx

AIN1

xxx

AIN1

000

ADC0

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