ATmega16 datasheet(86/315 Pages) ATMEL

86 / 315 page

ATmega16(L)

2466E–AVR–10/02

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

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.

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.

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;

...

ATMEGA16 Datenblatt(PDF) 86 Page - ATMEL Corporation

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