Datasheet

109
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 18-10. Timer/Counter Timing Diagram, Setting of OCF2, with Prescaler (f
clk_I/O
/8)
Figure 18-11 shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.
Figure 18-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Prescaler (f
clk_I/O
/8)
18.9 Asynchronous Operation of the Timer/Counter
18.9.1 Asynchronous Operation of Timer/Counter2
When Timer/Counter2 operates asynchronously, some considerations must be taken.
• Warning: When switching between asynchronous and synchronous clocking of Timer/Counter2, the Timer
Registers TCNT2, OCR2, and TCCR2 might be corrupted. A safe procedure for switching clock source is:
1. Disable the Timer/Counter2 interrupts by clearing OCIE2 and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2, and TCCR2.
4. To switch to asynchronous operation: Wait for TCN2UB, OCR2UB, and TCR2UB.
5. Clear the Timer/Counter2 Interrupt Flags.
6. Enable interrupts, if needed.
OCFn
OCRn
TCNTn
OCRn Value
OCRn - 1 OCRn OCRn + 1 OCRn + 2
clk
I/O
clk
Tn
(clk
I/O
/8)
OCFn
OCRn
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)