ATTINY817-MFR. datasheet(41/492 Pages) MICROCHIP

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8.5.6

Accessing 16-bit Registers

The AVR data bus is 8 bits wide, and 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 low byte of the 16-bit register must be written before the high byte. The low byte

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

temporary register is copied into the low 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 also be read and written directly from user software.

8.5.7

CCP - Configuration Change Protection

System critical I/O register settings are protected from accidental modification. Flash self-programming

(via store to NVM controller) is protected from accidental execution. This is handled globally by the

configuration change protection (CCP) register.

Changes to the protected I/O registers or bits, or execution of protected instructions, are only possible

after the CPU writes a signature to the CCP register. The different signatures are listed in the description

of the CCP register (CPU.CCP).

There are two modes of operation: one for protected I/O registers, and one for the protected self-

programming.

ATTINY817-MFR. Datasheet(PDF) 41 Page - Microchip Technology