Datasheet

5.2 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. A 16-bit register must be byte accessed using two read or write operations. The 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 the 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 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.
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
in r16,TCNT1L
in r17,TCNT1H
:.
C Code Example
(1)
unsigned int i;
:.
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
:.
Note: 1. See About Code Examples.
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 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 register, the main code must disable the interrupts during the 16-bit access.
The following code examples show how to do an atomic read of the TCNT1 Register contents. Reading
any of the OCR1A/B or ICR1 Registers can be done by using the same principle.
AVR 8-Bit Microcontroller
Pin Configurations
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 17