Datasheet

8197C–AVR–05/11

ATtiny261A/461A/861A

OCF0B, but in 16-bit mode the match can set only the Output Compare Flag OCF0A as there is

only one Output Compare Unit. If the corresponding interrupt is enabled, the Output Compare

Flag generates an Output Compare interrupt. The Output Compare Flag is automatically cleared

when the interrupt is executed. Alternatively, the flag can be cleared by software by writing a log-

ical one to its I/O bit location. Figure 11-5 shows a block diagram of the Output Compare unit.

Figure 11-5. Output Compare Unit, Block Diagram

11.6.1 Compare Match Blocking by TCNT0 Write

All CPU write operations to the TCNT0H/L Register will block any Compare Match that occur in

the next timer clock cycle, even when the timer is stopped. This feature allows OCR0A/B to be

initialized to the same value as TCNT0 without triggering an interrupt when the Timer/Counter

clock is enabled.

11.6.2 Using the Output Compare Unit

Since writing TCNT0H/L will block all Compare Matches for one timer clock cycle, there are risks

involved when changing TCNT0H/L when using the Output Compare Unit, independently of

whether the Timer/Counter is running or not. If the value written to TCNT0H/L equals the

OCR0A/B value, the Compare Match will be missed.

11.7 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is

defined by the Timer/Counter Width (TCW0), Input Capture Enable (ICEN0) and Wave Genera-

tion Mode (CTC0) bits. See “TCCR0A – Timer/Counter0 Control Register A” on page 83.

Table 11-3 summarises the different modes of operation.

11.7.1 Normal, 8-bit Mode

In Normal 8-bit mode (see Table 11-3), the counter (TCNT0L) is incrementing until it overruns

when it passes its maximum 8-bit value (MAX = 0xFF) and then restarts from the bottom (0x00).

OCFnx (Int.Req.)

= (8/16-bit Comparator )

OCRnx

DATA BUS

TCNTn

Table 11-3. Modes of operation

Mode ICEN0 TCW0 CTC0 Mode of Operation TOP Update of OCRx at TOV Flag Set on

0000Normal, 8-bit Mode 0xFF ImmediateMAX (0xFF)

1001CTC Mode, 8-bit OCR0A ImmediateMAX (0xFF)

2 0 1 X Normal, 16-bit Mode 0xFFFF Immediate MAX (0xFFFF)

3 1 0 X Input Capture Mode, 8-bit 0xFF Immediate MAX (0xFF)

4 1 1 X Input Capture Mode, 16-bit 0xFFFF Immediate MAX (0xFFFF)