Datasheet

Figure 22-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with
Prescaler (f
clk_I/O
/8)
OCFn
OCRn
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
22.9 Asynchronous Operation of the Timer/Counter
22.9.1 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.
• The Oscillator is optimized for use with a 32.768kHz watch crystal. Applying an external clock to the
TOSC1 pin may result in incorrect Timer/Counter2 operation. The CPU main clock frequency must
be more than four times the Oscillator frequency.
• When writing to one of the registers TCNT2, OCR2, or TCCR2, 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 three mentioned registers have their individual temporary register, which means that
e.g. writing to TCNT2 does not disturb an OCR2 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 mode after having written to TCNT2, OCR2, or TCCR2, 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 the Output Compare2 interrupt is used to wake up the device, since the Output
Compare function is disabled during writing to OCR2 or TCNT2. If the write cycle is not finished,
AVR 8-Bit Microcontroller
8-bit Timer/Counter2 with PWM and Asynchronous Ope...
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Datasheet Complete
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