ATtiny261-ESSZ datasheet(76/227 Pages) ATMEL

76 / 227 page

7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

Figure 14-2. CTC Mode, Timing Diagram

An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF0A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating

the TOP value. However, changing TOP to a value close to BOTTOM when the counter is run-

ning with none or a low prescaler value must be done with care since the CTC mode does not

have the double buffering feature. If the new value written to OCR0A is lower than the current

value of TCNT0, the counter will miss the Compare Match. The counter will then have to count to

its maximum value (0xFF) and wrap around starting at 0x00 before the Compare Match can

occur. As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle

that the counter counts from MAX to 0x00.

14.5.3

16-bit Mode

In 16-bit mode, see Table 14-3 on page 75, the counter (TCNT0H/L) is a incrementing until it

overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and then restarts from the

bottom (0x0000). The Overflow Flag (TOV0) will be set in the same timer clock cycle as the

TCNT0H/L becomes zero. The TOV0 Flag in this case behaves like a 17th bit, except that it is

only set, not cleared. However, combined with the timer overflow interrupt that automatically

clears the TOV0 Flag, the timer resolution can be increased by software. There are no special

cases to consider in the Normal mode, a new counter value can be written anytime. The Output

Compare Unit can be used to generate interrupts at some given time.

14.5.4

8-bit Input Capture Mode

The Timer/Counter0 can also be used in an 8-bit Input Capture mode, see Table 14-3 on page

75 for bit settings. For full description, see the section “Input Capture Unit” on page 77.

14.5.5

16-bit Input Capture Mode

The Timer/Counter0 can also be used in a 16-bit Input Capture mode, see Table 14-3 on page

75 for bit settings. For full description, see the section “Input Capture Unit” on page 77.

TCNTn

OCnx Interrupt Flag Set

Period

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