ATmega644PA datasheet(138/468 Pages) ATMEL

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138

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

A FOCnA/FOCnB strobe will not generate any interrupt nor will it clear the timer in Clear Timer

on Compare match (CTC) mode using OCRnA as TOP.

The FOCnA/FOCnB bits are always read as zero.

15.11.4

TCNT1H and TCNT1L –Timer/Counter1

The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct

access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To

ensure that both the high and low bytes are read and written simultaneously when the CPU

accesses these registers, the access is performed using an 8-bit temporary High Byte Register

(TEMP). This temporary register is shared by all the other 16-bit registers. See Section “15.3” on

page 115.

Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a com-

pare match between TCNTn and one of the OCRnx Registers.

Writing to the TCNTn Register blocks (removes) the compare match on the following timer clock

for all compare units.

15.11.5

OCR1AH and OCR1AL – Output Compare Register 1 A

15.11.6

OCR1BH and OCR1BL – Output Compare Register 1 B

The Output Compare Registers contain a 16-bit value that is continuously compared with the

counter value (TCNTn). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OCnx pin.

The Output Compare Registers are 16-bit in size. To ensure that both the high and low bytes are

written simultaneously when the CPU writes to these registers, the access is performed using an

8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other

16-bit registers. See Section “15.3” on page 115.

Bit

76543210

(0x85)

TCNT1[15:8]

TCNT1H

(0x84)

TCNT1[7:0]

TCNT1L

Read/Write

R/W

Initial Value

00000000

Bit

76543210

(0x89)

OCR1A[15:8]

OCR1AH

(0x88)

OCR1A[7:0]

OCR1AL

Read/Write

R/W

Initial Value

00000000

Bit

76543210

(0x8B)

OCR1B[15:8]

OCR1BH

(0x8A)

OCR1B[7:0]

OCR1BL

Read/Write

R/W

Initial Value

00000000

ATmega644PA Datasheet(PDF) 138 Page - ATMEL Corporation

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