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ATmega64A [DATASHEET]
8160D–AVR–02/2013
16.3
Accessing 16-bit Registers
The TCNTn, OCRnA/B/C, and ICRn are 16-bit registers that can be accessed by the AVR CPU via the 8-bit data
bus. The 16-bit register must be byte accessed using two read or write operations. Each 16-bit timer has a single 8-
bit register for temporary storing of the high byte of the 16-bit access. The same temporary register is shared
between all 16-bit registers within each 16-bit timer. Accessing the low byte triggers the 16-bit read or write opera-
tion. When the low byte of a 16-bit register is written by the CPU, the high byte stored in the temporary register, and
the low byte written are both copied into the 16-bit register in the same clock cycle. When the low byte of a 16-bit
register is read by the CPU, the high byte of the 16-bit register is copied into the temporary register in the same
clock cycle as the low byte is read.
Not all 16-bit accesses uses the temporary register for the high byte. Reading the OCRnA/B/C 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 OCRnA/B/C and ICRn Registers.
Note that when using “C”, the compiler handles the 16-bit access.
Note:
The assembly code example returns the TCNTn 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.
:.
; Set TCNT
n to 0x01FF
ldi
r17,0x01
ldi
r16,0xFF
out
TCNT
nH,r17
out
TCNT
nL,r16
; Read TCNT
n into r17:r16
in
r16,TCNT
nL
in
r17,TCNT
nH
:.
unsigned int
i;
:.
/* Set TCNT
n to 0x01FF */
TCNT
n = 0x1FF;
/* Read TCNT
n into i */
i = TCNT
n;
:.