Datasheet

184
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
Figure 20-10 shows the setting of OCF2A in all modes except CTC mode.
Figure 20-10. Timer/Counter Timing Diagram, Setting of OCF2A, with Prescaler (f
clk_I/O
/8)
Figure 20-11 shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.
Figure 20-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Pres-
caler (f
clk_I/O
/8)
20.8 Asynchronous Operation of Timer/Counter2
When Timer/Counter2 operates asynchronously, some considerations must be taken.
• Warning: W hen switching between asynchronous and synchronous clocking of
Timer/Counter2, the Timer Registers TCN T2, OCR2x, and TCCR2x might be corrupted. A
safe procedure for switching clock source is:
1. Disable the Timer/Counter2 interrupts by clearing OCIE2x and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2x, and TCCR2x.
4. To switch to asynchronous operation: Wait for TCN2UB, OCR2xUB, and TCR2xUB.
5. Clear the Timer/Counter2 Interrupt Flags.
6. Enable interrupts, if needed.
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clk
I/O
clk
Tn
(clk
I/O
/8)
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)