Data Sheet

115
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
Note: 1. See ”About Code Examples” on page 8.
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 TCNT1 value in the r17:r16 register pair.
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 TCNT1 Register contents. Reading any of
the OCR1A/B or ICR1 Registers can be done by using the same principle.
Assembly Code Examples
(1)
...
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
...
C Code Examples
(1)
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
...