ATmega162 datasheet(109/324 Pages) ATMEL

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109

2513K–AVR–07/09

ATmega162/V

Accessing 16-bit

Registers

The TCNTn, OCRnA/B, 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 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 tempo-

rary 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 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 and ICRn Registers. Note that when using “C”, the compiler handles the 16-bit

access.

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.

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 Regis-

ters, 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.

Assembly Code Examples

(1)

...

; 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

r16,TCNT

r17,TCNT

...

C Code Examples(1)

unsigned int

...

/* Set TCNT

n to 0x01FF */

TCNT

n = 0x1FF;

/* Read TCNT

n into i */

i = TCNT

...

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