100
ATmega64A [DATASHEET]
8160D–AVR–02/2013
When writing to one of the registers TCNT0, OCR0, or TCCR0, 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, for example, writing to TCNT0 does not disturb an OCR0 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 TCNT0, OCR0, or TCCR0, the
user must wait until the written register has been updated if Timer/Counter0 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 Compare0 interrupt is used to wake up the device, since the Output Compare function is disabled during
writing to OCR0 or TCNT0. If the write cycle is not finished, and the MCU enters sleep mode before the
OCR0UB bit returns to zero, the device will never receive a Compare Match interrupt, and the MCU will not
wake up.
If Timer/Counter0 is used to wake the device up from Power-save or Extended Standby mode, precautions must
be taken if the user wants to reenter 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 TCCR0, TCNT0, or OCR0.
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 32.768 kHz Oscillator for Timer/Counter0 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/Counter0 after Power-up or wake-up from
Power-down or Standby mode. The contents of all Timer/Counter0 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 TCNT0 Register shortly after wake-up from Power-save may give an incorrect result. Since
TCNT0 is clocked on the asynchronous TOSC clock, reading TCNT0 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, TCNT0 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 TCNT0 is thus as follows:
1.
Write any value to either of the registers OCR0 or TCCR0.
2.
Wait for the corresponding Update Busy Flag to be cleared.
3.
Read TCNT0.
During asynchronous operation, the synchronization of the interrupt flags for the asynchronous timer takes
three processor cycles plus one timer cycle. The timer is therefore advanced by at least one before the
processor can read the timer value causing the setting of the interrupt flag. The Output Compare pin is changed
on the timer clock and is not synchronized to the processor clock.