Read back protection has nothing to do with the DFU as the readback only protect the content of flash (code region 0 or all) from reading from the SWD interface. DFU on the other hand is the modification of the flash by the CPU (by the code in bootloader and the MBR).
If CLENR0 = 0xFFFFFFFF, it's interpreted as CLENR0 size = 0. Everything is Region 1.
When you set CLENR0, (e.g enable protection when flashing softdevice), it's still possible to update the softdevice (region 0) because the MBR is located inside region 0 and it's possible to modify code region 0 (but not changing the size of it).
Read back protection has nothing to do with the DFU as the readback only protect the content of flash (code region 0 or all) from reading from the SWD interface. DFU on the other hand is the modification of the flash by the CPU (by the code in bootloader and the MBR).
If CLENR0 = 0xFFFFFFFF, it's interpreted as CLENR0 size = 0. Everything is Region 1.
When you set CLENR0, (e.g enable protection when flashing softdevice), it's still possible to update the softdevice (region 0) because the MBR is located inside region 0 and it's possible to modify code region 0 (but not changing the size of it).
