ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 94

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

14.3
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Accessing 16-bit Registers
Atmel ATtiny24/44/84 [Preliminary]
TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the Atmel
CPU via the 8-bit data bus. The 16-bit registers must be byte accessed using two read or write
operations. Each 16-bit timer has a single 8-bit register for temporary storage 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 regis-
ter 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 use 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 access-
ing the OCR1A/B and ICR1 registers. Note that when using C, the compiler handles the 16-bit
access.
Note:
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
Assembly Code Examples
C Code Examples
...
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
...
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
...
1. See
“About Code Examples” on page
(1)
(1)
6.
7701E–AVR–02/11
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AVR
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