Rainbow Electronics ATtiny861_V - Manual (Page 75)

75
2588B–AVR–11/06
ATtiny261/461/861
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 74 , 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
74 for bit settings. For full description, see the section ”Input Capture Unit” on page 76 .
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
74 for bit settings. For full description, see the section ”Input Capture Unit” on page 76 .
TCNTn
OCnx Interrupt Flag Set
1 4 Period 2 3