ATmega16U4 datasheet(161/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

It is important to notice that accessing 10-bit registers are atomic operations. If an interrupt

occurs between the two instructions accessing the 10-bit register, and the interrupt code

updates the TC4H register by accessing the same or any other of the 10-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 TC4H 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/D registers can be done by using the same principle.

Note:

1. The example code assumes that the part specific header file is included.

For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

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

Assembly Code Example

TIM1_ReadTCNT

; Save global interrupt flag

r18,SREG

; Disable interrupts

cli

; Read TCNT

n into r17:r16

r16,TCNT

r17,TC

; Restore global interrupt flag

out

SREG,r18

ret

C Code Example

unsigned int

TIM1_ReadTCNTn( void )

{

unsigned char

sreg;

unsigned int

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Read TCNTn into i */

i = TCNTn;

i |= ((unsigned int)TCnH << 8);

/* Restore global interrupt flag

SREG = sreg;

return

}

ATmega16U4 Datasheet(PDF) 161 Page - ATMEL Corporation

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