ATmega325P datasheet(152/364 Pages) ATMEL

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152

8023F–AVR–07/09

ATmega325P/3250P

• Bit 4 – EXCLK: Enable External Clock Input

When EXCLK is written to one, and asynchronous clock is selected, the external clock input buf-

fer is enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a

32 kHz crystal. Writing to EXCLK should be done before asynchronous operation is selected.

Note that the crystal Oscillator will only run when this bit is zero.

• Bit 3 – AS2: Asynchronous Timer/Counter2

When AS2 is written to zero, Timer/Counter2 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, OCR2A, and

TCCR2A 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 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 0 – TCR2UB: 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.

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, OCR2A, and TCCR2A are different. When reading

TCNT2, the actual timer value is read. When reading OCR2A or TCCR2A, the value in the tem-

porary storage register is read.

16.10.5

TIMSK2 – Timer/Counter2 Interrupt Mask Register

• Bit 1 – OCIE2A: Timer/Counter2 Output Compare Match A Interrupt Enable

When the OCIE2A bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Compare Match A interrupt is enabled. The corresponding interrupt is executed

if a compare match in Timer/Counter2 occurs, i.e., when the OCF2A bit is set in the Timer/Coun-

ter 2 Interrupt Flag Register – TIFR2.

Bit

65432

(0x70)

–

OCIE2A

TOIE2

TIMSK2

Read/Write

RRRR

R/W

Initial Value

00000

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