ATtiny48 datasheet(86/266 Pages) ATMEL

86 / 266 page

8008C–AVR–03/09

ATtiny48/88

12.3

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 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 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.

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 TCNT1 value in the r17:r16 register pair.

Assembly Code Examples(1)

...

; Set TCNT

1 to 0x01FF

ldi

r17,0x01

ldi

r16,0xFF

out

TCNT

1H,r17

out

TCNT

1L,r16

; Read TCNT

1 into r17:r16

r16,TCNT

r17,TCNT

...

C Code Examples(1)

unsigned int

...

/* Set TCNT

1 to 0x01FF */

TCNT

1 = 0x1FF;

/* Read TCNT

1 into i */

i = TCNT

...

ATtiny48 Datasheet(PDF) 86 Page - ATMEL Corporation

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