35.5.6 Register Access Protection

All registers with write-access are optionally write-protected by the Peripheral Access Controller (PAC), except the following registers:

• The Control A (CTRLA) register
• The Transmit Data (TXDATA) register
• The Interrupt Flag Status and Clear (INTFLAG) register
• The Interrupt Flag Status and Clear (INTFLAG) register

PAC write-protection is denoted by the `'PAC Write-Protection' property in the register description. Write-protection does not apply to accesses through an external debugger.