29.6.7 Secure Access Rights

Non-secure write to CTRLA register or DPRESCALER register is prohibited. Non-secure read to CTRLA or DPRESCALER register or SYNCBUSY register will return zero with no error resulting. Non-secure write to a bit of EVCTRL, ASYNCH, DEBOUNCEN, INTENCLR, INTENSET, INTFLAG and CONFIG registers is prohibited if the related bit of NONSEC.EXTINT is zero. Non-secure write to NMICTRL and NMIFLAG registers is prohibited if NONSECNMI.NMI is zero. Bits relating to secure EXTINT read as zero in non-secure mode with no error resulting.