ATtiny25-15ST datasheet(76/196 Pages) ATMEL

76 / 196 page

7598H–AVR–07/09

ATtiny25/45/85

A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR0B as TOP.

The FOC0B bit is always read as zero.

• Bits 5:4 – Res: Reserved Bits

These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.

• Bit 3 – WGM02: Waveform Generation Mode

See the description in the “Timer/Counter Control Register A – TCCR0A” on page 72.

• Bits 2:0 – CS02:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter.

If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the

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

counting.

12.8.3

Timer/Counter Register – TCNT0

The Timer/Counter Register gives direct access, both for read and write operations, to the

Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare

Match on the following timer clock. Modifying the counter (TCNT0) while the counter is running,

introduces a risk of missing a Compare Match between TCNT0 and the OCR0x Registers.

12.8.4

Output Compare Register A – OCR0A

The Output Compare Register A contains an 8-bit value that is continuously compared with the

counter value (TCNT0). A match can be used to generate an Output Compare interrupt, or to

generate a waveform output on the OC0A pin.

Table 12-8.

Clock Select Bit Description

CS02

CS01

CS00

Description

No clock source (Timer/Counter stopped)

001

clkI/O/(No prescaling)

010

clkI/O/8 (From prescaler)

011

clkI/O/64 (From prescaler)

100

clkI/O/256 (From prescaler)

101

clkI/O/1024 (From prescaler)

External clock source on T0 pin. Clock on falling edge.

External clock source on T0 pin. Clock on rising edge.

Bit

765

432

TCNT0[7:0]

TCNT0

Read/Write

R/W

Initial Value

Bit

765

432

OCR0A[7:0]

OCR0A

Read/Write

R/W

Initial Value

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