Datasheet

179
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
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 Prescaler (f
clk_I/O
/8)
20.8 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, 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.
• The CPU main clock frequency must be more than four times the Oscillator frequency.
• When writing to one of the registers TCNT2, OCR2x, or TCCR2x, the value is transferred to a temporary
register, and latched after two positive edges on TOSC1. The user should not write a new value before the
contents of the temporary register have been transferred to its destination. Each of the five mentioned registers
have their individual temporary register, which means that, for example, writing to TCNT2 does not disturb an
OCR2x write in progress. To detect that a transfer to the destination register has taken place, the
Asynchronous Status Register – ASSR has been implemented.
• When entering Power-save or ADC Noise Reduction mode after having written to TCNT2, OCR2x, or TCCR2x,
the user must wait until the written register has been updated if Timer/Counter2 is used to wake up the device.
Otherwise, the MCU will enter sleep mode before the changes are effective. This is particularly important if any
of the Output Compare2 interrupt is used to wake up the device, since the Output Compare function is disabled
during writing to OCR2x or TCNT2. If the write cycle is not finished, and the MCU enters sleep mode before the
corresponding OCR2xUB bit returns to zero, the device will never receive a compare match interrupt, and the
MCU will not wake up.
• If Timer/Counter2 is used to wake the device up from Power-save or ADC Noise Reduction mode, precautions
must be taken if the user wants to re-enter one of these modes: The interrupt logic needs one TOSC1 cycle to
be reset. If the time between wake-up and re-entering sleep mode is less than one TOSC1 cycle, the interrupt
will not occur, and the device will fail to wake up. If the user is in doubt whether the time before re-entering
Power-save or ADC Noise Reduction mode is sufficient, the following algorithm can be used to ensure that one
TOSC1 cycle has elapsed:
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)