Datasheet

12.6 Register Description

12.6.1 Accessing 16-Bit Registers

The AVR data bus is 8-bits wide, so accessing 16-bit registers requires atomic operations. These

registers must be byte-accessed using two read or write operations. 16-bit registers are connected to the

8-bit bus and a temporary register using a 16-bit bus.

For a write operation, the high byte of the 16-bit register must be written before the low byte. The high

byte is then written into the temporary register. When the low byte of the 16-bit register is written, the

temporary register is copied into the high byte of the 16-bit register in the same clock cycle.

For a read operation, the low byte of the 16-bit register must be read before the high byte. When the low

byte 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. When the high byte is read, it is then read from the

temporary register.

This ensures that the low and high bytes of 16-bit registers are always accessed simultaneously when

reading or writing the register.

Interrupts can corrupt the timed sequence if an interrupt is triggered and accesses the same 16-bit

writing or reading 16-bit registers.

The temporary registers can be read and written directly from user software.

Note: For more information, refer to Accessing 16-bit Timer/Counter registers.