ATmega16A datasheet(91/352 Pages) ATMEL

91 / 352 page

8154B–AVR–07/09

ATmega16A

16.3

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.

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 tem-

porary 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.

Note:

1. See “About Code Examples” on page 7.

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 Regis-

ters, then the result of the access outside the interrupt will be corrupted. Therefore, when both

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

r16,TCNT1L

r17,TCNT1H

C Code Example(1)

unsigned int

/* Set TCNT1 to 0x01FF */

TCNT

1 = 0x1FF;

/* Read TCNT1 into i */

i = TCNT1;

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