ATmega325P datasheet(132/364 Pages) ATMEL

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132

8023F–AVR–07/09

ATmega325P/3250P

A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer

on Compare match (CTC) mode using OCR1A as TOP. The FOC1A/FOC1B bits are always

read as zero.

15.11.4

TCNT1H and TCNT1L – Timer/Counter1

The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) 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 110.

Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a com-

pare match between TCNT1 and one of the OCR1x Registers.

Writing to the TCNT1 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 (TCNT1). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC1x 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 110.

Bit

(0x85)

TCNT1[15:8]

TCNT1H

(0x84)

TCNT1[7:0]

TCNT1L

Read/Write

R/W

Initial Value

Bit

543210

(0x89)

OCR1A[15:8]

OCR1AH

(0x88)

OCR1A[7:0]

OCR1AL

Read/Write

R/W

Initial Value

000000

Bit

765

4321

(0x8B)

OCR1B[15:8]

OCR1BH

(0x8A)

OCR1B[7:0]

OCR1BL

Read/Write

R/W

Initial Value

000

0000

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