ATmega64A datasheet(115/392 Pages) ATMEL

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115

8160C–AVR–07/09

ATmega64A

It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt

occurs between the two instructions accessing the 16-bit register, and the interrupt code

updates the temporary register by accessing the same or any other of the 16-bit timer registers,

then the result of the access outside the interrupt will be corrupted. Therefore, when both the

main code and the interrupt code update the temporary register, the main code must disable the

interrupts during the 16-bit access.

The following code examples show how to do an atomic read of the TCNTn Register contents.

Reading any of the OCRnA/B/C or ICRn Registers can be done by using the same principle.

Note:

1. See “About Code Examples” on page 8.

The assembly code example returns the TCNTn value in the r17:r16 register pair.

Assembly Code Example

(1)

TIM16_ReadTCNT

; Save global interrupt flag

r18,SREG

; Disable interrupts

cli

; Read TCNT

n into r17:r16

r16,TCNT

r17,TCNT

; Restore global interrupt flag

out

SREG,r18

ret

C Code Example(1)

unsigned int

TIM16_ReadTCNT

n( void )

{

unsigned char

sreg;

unsigned int

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Read TCNT

n into i */

i = TCNT

/* Restore global interrupt flag */

SREG = sreg;

return

}

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