Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
134
15.9 Asynchronous operation of the Timer/Counter
15.9.1 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, OCR2x, and TCCR2x might be corrupted. A safe procedure for switching clock source is:
a. Disable the Timer/Counter2 interrupts by clearing OCIE2x and TOIE2.
b. Select clock source by setting AS2 as appropriate.
c. Write new values to TCNT2, OCR2x, and TCCR2x.
d. To switch to asynchronous operation: Wait for TCN2xUB, OCR2xUB, and TCR2xUB.
e. Clear the Timer/Counter2 interrupt flags.
f. 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 e.g. 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:
a. Write a value to TCCR2x, TCNT2, or OCR2x.
b. Wait until the corresponding update busy flag in ASSR returns to zero.
c. Enter power-save or ADC noise reduction 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 ADC noise reduction 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.