ATmega16 datasheet(131/357 Pages) ATMEL

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2466S–AVR–05/09

ATmega16(L)

Asynchronous

Operation of the

Timer/Counter

Asynchronous Status

• Bit 3 – AS2: Asynchronous Timer/Counter2

When AS2 is written to zero, Timer/Counter 2 is clocked from the I/O clock, clk

I/O. When AS2 is

written to one, Timer/Counter2 is clocked from a Crystal Oscillator connected to the Timer Oscil-

lator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of TCNT2, OCR2, and

TCCR2 might be corrupted.

• Bit 2 – TCN2UB: Timer/Counter2 Update Busy

When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set.

When TCNT2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that TCNT2 is ready to be updated with a new value.

• Bit 1 – OCR2UB: Output Compare Register2 Update Busy

When Timer/Counter2 operates asynchronously and OCR2 is written, this bit becomes set.

When OCR2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2 is ready to be updated with a new value.

• Bit 0 – TCR2UB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2 is written, this bit becomes set.

When TCCR2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that TCCR2 is ready to be updated with a new value.

If a write is performed to any of the three 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, OCR2, and TCCR2 are different. When reading TCNT2,

the actual timer value is read. When reading OCR2 or TCCR2, the value in the temporary stor-

age register is read.

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, OCR2, and TCCR2 might be corrupted. A

safe procedure for switching clock source is:

Disable the Timer/Counter2 interrupts by clearing OCIE2 and TOIE2.

Select clock source by setting AS2 as appropriate.

Write new values to TCNT2, OCR2, and TCCR2.

To switch to asynchronous operation: Wait for TCN2UB, OCR2UB, and TCR2UB.

Clear the Timer/Counter2 Interrupt Flags.

Enable interrupts, if needed.

Bit

765

–

AS2

TCN2UB

OCR2UB

TCR2UB

ASSR

Read/Write

RRRR

R/W

Initial Value

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