Datasheet

121
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
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
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 32.768 kHz 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 for example that 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 or Extended Standby 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, and the MCU enters sleep mode before the
OCR2UB 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 Extended Standby 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-
OCFn
OCRn
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)