ATmega88 datasheet(131/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

If external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

14.10.3

Timer/Counter1 Control Register C – TCCR1C

• Bit 7 – FOC1A: Force Output Compare for Channel A

• Bit 6 – FOC1B: Force Output Compare for Channel B

The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.

However, for ensuring compatibility with future devices, these bits must be set to zero when

TCCR1A is written when operating in a PWM mode. When writing a logical one to the

FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit.

The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the

FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the

COM1x1:0 bits that determine the effect of the forced compare.

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.

14.10.4

Timer/Counter1 – TCNT1H and TCNT1L

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 “Accessing 16-bit

Registers” on page 107.

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.

Bit

210

FOC1A

FOC1B

–

TCCR1C

Read/Write

R/W

Initial Value

Bit

76543210

TCNT1[15:8]

TCNT1H

TCNT1[7:0]

TCNT1L

Read/Write

R/W

Initial Value

00000000

ATmega88 Datasheet(PDF) 131 Page - ATMEL Corporation

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