Read-While-Write and No Read-While-Write Flash Sections

Whether the CPU supports Read-While-Write (RWW) or if the CPU is halted during a boot loader software update is dependent on which address that is being programmed. In addition to the two sections that are configurable by the BOOTSZ fuses as described above, the Flash is also divided into two fixed sections; the RWW section and the No Read-While-Write (NRWW) section. The limit between the RWW and NRWW sections is given in the Boot Loader Parameters section and Figure 2. The main differences between the two sections are:

• When erasing or writing a page located inside the RWW section, the NRWW section can be read during the operation
• When erasing or writing a page located inside the NRWW section, the CPU is halted during the entire operation

The user software can never read any code that is located inside the RWW section during a boot loader software operation. The syntax “Read-While-Write section” refers to which section that is being programmed (erased or written), not which section that actually is being read during a boot loader software update.