4.4 Registers and Bits

Table 4-4. Register and Bit Mnemonics
Symbol	Description
R/W	Read/Write accessible register bit. The user can read from and write to this bit.
R	Read-only accessible register bit. The user can only read this bit⁽¹⁾. Writes will be ignored.
W	Write-only accessible register bit. The user can only write this bit. Reading this bit will return an undefined value.
BITFIELD	Bit field names are shown in uppercase. Example: INTMODE.
BITFIELD[n:m]	A set of bits from bit n down to m. Example: PINA[3:0] = {PINA3, PINA2, PINA1, PINA0}.
Reserved	Reserved bits, bit fields, and bit field values are unused and reserved for future use. For compatibility with future devices, always write reserved bits to ‘`0`’ when the register is written. Reserved bits will always return zero when read.
PERIPHERALn	If several instances of the peripheral exist, the peripheral name is followed by a single number to identify one instance. Example: USARTn is the collection of all instances of the USART module, while USART3 is one specific instance of the USART module.
PERIPHERALx	If several instances of the peripheral exist, the peripheral name is followed by a single capital letter (A-Z) to identify one instance. Example: PORTx is the collection of all instances of the PORT module, while PORTB is one specific instance of the PORT module.
Reset	Value of a register after a Power-on Reset. This is also the value of registers in a peripheral after performing a software Reset of the peripheral, except for the Debug Control registers.
SET/CLR/TGL	Registers with SET/CLR/TGL suffix allow the user to clear and set bits in a register without doing a read-modify-write operation. Each SET/CLR/TGL register is paired with the register it is affecting. Both registers in a register pair return the same value when read. Example: In the PORT peripheral, the OUT and OUTSET registers form such a register pair. The contents of OUT will be modified by a write to OUTSET. Reading OUT and OUTSET will return the same value. Writing a ‘`1`’ to a bit in the CLR register will clear the corresponding bit in both registers. Writing a ‘`1`’ to a bit in the SET register will set the corresponding bit in both registers. Writing a ‘`1`’ to a bit in the TGL register will toggle the corresponding bit in both registers.

Note:

Fuses can be read by the user application, but they cannot be written during application run time. Fuses can only be programmed by the UPDI unless other protective mechanisms are available and activated.