Datasheet

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,
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-save or Extended Standby mode is
sufficient, the following algorithm can be used to ensure that one TOSC1 cycle has elapsed:
1. Write a value to TCCR2, TCNT2, or OCR2.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save or Extended Standby mode.
• When the asynchronous operation is selected, the 32.768kHz Oscillator for Timer/Counter2 is
always running, except in Power-down and Standby modes. After a Power-up Reset or Wake-up
from Power-down or Standby mode, the user should be aware of the fact that this Oscillator might
take as long as one second to stabilize. The user is advised to wait for at least one second before
using Timer/Counter2 after Power-up or Wake-up from Power-down or Standby mode. The
contents of all Timer/Counter2 Registers must be considered lost after a wake-up from Power-down
or Standby mode due to unstable clock signal upon start-up, no matter whether the Oscillator is in
use or a clock signal is applied to the TOSC1 pin.
• Description of wake up from Power-save or Extended Standby mode when the timer is clocked
asynchronously: When the interrupt condition is met, the wake up process is started on the
following cycle of the timer clock, that is, the timer is always advanced by at least one before the
processor can read the counter value. After wake-up, the MCU is halted for four cycles, it executes
the interrupt routine, and resumes execution from the instruction following SLEEP.
• Reading of the TCNT2 Register shortly after wake-up from Power-save may give an incorrect
result. Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2 must be done
through a register synchronized to the internal I/O clock domain. Synchronization takes place for
every rising TOSC1 edge. When waking up from Power-save mode, and the I/O clock (clk
I/O
) again
becomes active, TCNT2 will read as the previous value (before entering sleep) until the next rising
TOSC1 edge. The phase of the TOSC clock after waking up from Power-save mode is essentially
unpredictable, as it depends on the wake-up time. The recommended procedure for reading
TCNT2 is thus as follows:
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
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