ATmega8(L)
Accessing 16-bit
Registers
The TCNT1, OCR1A/B, and ICR1 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. The 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 the 16-bit timer. Accessing the Low byte triggers the 16-bit read or write
operation. 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
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.
Assembly Code Example(1)
...
; Set TCNT1 to 0x01FF
ldir17,0x01
ldir16,0xFF
outTCNT1H,r17
outTCNT1L,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;
...
Note:
1. The example code assumes that the part specific header file is included.
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 inter-
rupt 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 regis-
ter, the main code must disable the interrupts during the 16-bit access.
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2486M–AVR–12/03
ATMEL [ ATMEL ]
