AT90SCR100 datasheet(151/433 Pages) ATMEL

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TPR0327AY–SMS–30Jan09

AT90SCR100

When Timer/Counter2 operates asynchronously and OCR2A is written, this bit becomes set.

When OCR2A has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2A is ready to be updated with a new value.

• Bit 2 – OCR2BUB: Output Compare Register2 Update Busy

When Timer/Counter2 operates asynchronously and OCR2B is written, this bit becomes set.

When OCR2B has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2B is ready to be updated with a new value.

• Bit 1 – TCR2AUB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2A is written, this bit becomes set.

When TCCR2A has been updated from the temporary storage register, this bit is cleared by

hardware. A logical zero in this bit indicates that TCCR2A is ready to be updated with a new

value.

• Bit 0 – TCR2BUB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2B is written, this bit becomes set.

When TCCR2B has been updated from the temporary storage register, this bit is cleared by

hardware. A logical zero in this bit indicates that TCCR2B is ready to be updated with a new

value.

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.

13.3.9.2

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:

1. Disable the Timer/Counter2 interrupts by clearing OCIE2x and TOIE2.

2. Select clock source by setting AS2 as appropriate.

3. Write new values to TCNT2, OCR2x, and TCCR2x.

4. To switch to asynchronous operation: Wait for TCN2UB, OCR2xUB, and TCR2xUB.

5. Clear the Timer/Counter2 Interrupt Flags.

6. 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

AT90SCR100 데이터시트(PDF) 151 Page - ATMEL Corporation

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