AT90USB64/128
If a write is performed to any of the five Timer/Counter2 Registers while its update busy flag is
set, the updated value might get corrupted and cause an unintentional interrupt to occur.
The mechanisms for reading TCNT2, OCR2A, OCR2B, TCCR2A and TCCR2B are different.
When reading TCNT2, the actual timer value is read. When reading OCR2A, OCR2B, TCCR2A
and TCCR2B the value in the temporary storage register is read.
15.9.2
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 TCN2UB, 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 32.768 kHz Oscillator for Timer/Counter2
is always running, except in Power-down and Standby modes. After a Power-up Reset or
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7593A–AVR–02/06
ATMEL [ ATMEL ]
