Datasheet

76
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Not all 16-bit accesses uses the temporary register for the High byte. Reading the OCR1A/B 16-bit registers does
not involve using the temporary register.
To do a 16-bit write, the High byte must be written before the Low byte. For a 16-bit read, the Low byte must be
read before the High byte.
The following code examples show how to access the 16-bit Timer Registers assuming that no interrupts updates
the temporary register. The same principle can be used directly for accessing the OCR1A/B and ICR1 Registers.
Note that when using “C”, the compiler handles the 16-bit access.
Note: 1. See “About Code Examples” on page 6.
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 dis-
able the interrupts during the 16-bit access.
Assembly Code Example
(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 Example
(1)
unsigned int i;
:.
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
:.